Lift JSON library from clang-tools-extra/clangd to llvm/Support.

Summary:
This consists of four main parts:
- an type json::Expr representing JSON values of dynamic kind, which can be
composed,

Lift JSON library from clang-tools-extra/clangd to llvm/Support.

Summary:
This consists of four main parts:
- an type json::Expr representing JSON values of dynamic kind, which can be
composed, inspected, and modified
- a JSON parser from string -> json::Expr
- a JSON printer from json::Expr -> string, with optional pretty-printing
- a convention for mapping json::Expr <=> native types (fromJSON/toJSON)
Mapping functions are provided for primitives (e.g. int, vector) and the
ObjectMapper helper helps implement fromJSON for struct/object types.

Based on clangd's usage, a couple of places I'd appreciate review attention:
- fromJSON returns only bool. A richer error-signaling mechanism may be useful
to provide useful messages, or let recursive fromJSONs (containers/structs)
do careful error recovery.
- should json::obj be always explicitly written (like json::ary)
- there's no streaming parse API. I suspect there are some simple wins like
a callback API where the document is a long array, and each element is small.
But this can probably be bolted on easily when we see the need.

Reviewers: bkramer, labath

Subscribers: mgorny, ilya-biryukov, ioeric, MaskRay, llvm-commits

Differential Revision: https://reviews.llvm.org/D45753

llvm-svn: 336534

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