Reapply "[NFC] Explicitly pass a VFS when creating DiagnosticsEngine (#115852)"

This reverts commit a1153cd6fedd4c906a9840987934ca4712e34cb2 with fixes
to lldb breakages.

Fixes https://github.com/l

Reapply "[NFC] Explicitly pass a VFS when creating DiagnosticsEngine (#115852)"

This reverts commit a1153cd6fedd4c906a9840987934ca4712e34cb2 with fixes
to lldb breakages.

Fixes https://github.com/llvm/llvm-project/issues/117145.

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Revert "[NFC] Explicitly pass a VFS when creating DiagnosticsEngine (#115852)"

Reverted for causing:
https://github.com/llvm/llvm-project/issues/117145

This reverts commit bdd10d9d249bd1c2a45e3de56

Revert "[NFC] Explicitly pass a VFS when creating DiagnosticsEngine (#115852)"

Reverted for causing:
https://github.com/llvm/llvm-project/issues/117145

This reverts commit bdd10d9d249bd1c2a45e3de56a5accd97e953458.

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[NFC] Explicitly pass a VFS when creating DiagnosticsEngine (#115852)

Starting with 41e3919ded78d8870f7c95e9181c7f7e29aa3cc4 DiagnosticsEngine
creation might perform IO. It was implicitly defaultin

[NFC] Explicitly pass a VFS when creating DiagnosticsEngine (#115852)

Starting with 41e3919ded78d8870f7c95e9181c7f7e29aa3cc4 DiagnosticsEngine
creation might perform IO. It was implicitly defaulting to
getRealFileSystem. This patch makes it explicit by pushing the decision
making to callers.

It uses ambient VFS if one is available, and keeps using
`getRealFileSystem` if there aren't any VFS.

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[include-cleaner] Pass WorkingDir to suggestPathToFileForDiagnostics (#95114)

Addresses https://github.com/llvm/llvm-project/issues/81215.

[IncludeCleaner][clangd] Mark umbrella headers as users of private

Private headers inside umbrella files shouldn't be marked as unused.

Differential Revision: https://reviews.llvm.org/D146406

[include-cleaner] Add include-cleaner tool, with initial HTML report

The immediate goal is to start producing an HTML report to debug and explain
include-cleaner recommendations.
For now, this inclu

[include-cleaner] Add include-cleaner tool, with initial HTML report

The immediate goal is to start producing an HTML report to debug and explain
include-cleaner recommendations.
For now, this includes only the lowest-level piece: a list of the references
found in the source code.

How this fits into future ideas:
- under refs we can also show the headers providing the symbol, which includes
match those headers etc
- we can also annotate the #include lines with which symbols they cover, and
add whichever includes we're suggesting too
- the include-cleaner tool will likely have modes where it emits diagnostics
and/or applies edits, so the HTML report is behind a flag

Differential Revision: https://reviews.llvm.org/D135956

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[include-cleaner] Include-cleaner library structure, and simplistic AST walking.

Include-cleaner is a library that uses the clang AST and preprocessor to
determine which headers are used. It will be

[include-cleaner] Include-cleaner library structure, and simplistic AST walking.

Include-cleaner is a library that uses the clang AST and preprocessor to
determine which headers are used. It will be used in clang-tidy, in
clangd, in a standalone tool at least for testing, and in out-of-tree tools.

Roughly, it walks the AST, finds referenced decls, maps these to
used sourcelocations, then to FileEntrys, then matching these against #includes.
However there are many wrinkles: dealing with macros, standard library
symbols, umbrella headers, IWYU directives etc.

It is not built on the C++20 modules concept of usage, to allow:
- use with existing non-modules codebases
- a flexible API embeddable in clang-tidy, clangd, and other tools
- avoiding a chicken-and-egg problem where include cleanups are needed
before modules can be adopted

This library is based on existing functionality in clangd that provides
an unused-include warning. However it has design changes:
- it accommodates diagnosing missing includes too (this means tracking
where references come from, not just the set of targets)
- it more clearly separates the different mappings
(symbol => location => header => include) for better testing
- it handles special cases like standard library symbols and IWYU directives
more elegantly by adding unified Location and Header types instead of
side-tables
- it will support some customization of policy where necessary (e.g.
for style questions of what constitutes a use, or to allow
both missing-include and unused-include modes to be conservative)

This patch adds the basic directory structure under clang-tools-extra
and a skeleton version of the AST traversal, which will be the central
piece.
A more end-to-end prototype is in https://reviews.llvm.org/D122677

RFC: https://discourse.llvm.org/t/rfc-lifting-include-cleaner-missing-unused-include-detection-out-of-clangd/61228

Differential Revision: https://reviews.llvm.org/D124164

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[Testing] TestAST, a helper for writing straight-line AST tests

Tests that need ASTs have to deal with the awkward control flow of
FrontendAction in some way. There are a few idioms used:
- don't b

[Testing] TestAST, a helper for writing straight-line AST tests

Tests that need ASTs have to deal with the awkward control flow of
FrontendAction in some way. There are a few idioms used:
- don't bother with unit tests, use clang -dump-ast
- create an ASTConsumer by hand, which is bulky
- use ASTMatchFinder - works pretty well if matchers are actually
needed, very strange if they are not
- use ASTUnit - this yields nice straight-line code, but ASTUnit is a
terrifically complicated library not designed for this purpose

TestAST provides a very simple way to write straight-line tests: specify
the code/flags and it provides an AST that is kept alive until the
object is destroyed.
It's loosely modeled after TestTU in clangd, which we've successfully
used for a variety of tests.

I've updated a couple of clang tests to use this helper, IMO they're clearer.

Differential Revision: https://reviews.llvm.org/D123668

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