[clang] Refactor attr diagnostics to use %select (#122473)

A cleanup follow-up to #118501 and #118567.

[clang] Add Parse and Sema support for RegularKeyword attributes

This patch adds the Parse and Sema support for RegularKeyword attributes,
following on from a previous patch that added Attr.td suppo

[clang] Add Parse and Sema support for RegularKeyword attributes

This patch adds the Parse and Sema support for RegularKeyword attributes,
following on from a previous patch that added Attr.td support.

The patch is quite large. However, nothing outside the tests is
specific to the first RegularKeyword attribute (__arm_streaming).
The patch should therefore be a one-off, up-front cost. Other
attributes just need an entry in Attr.td and the usual Sema support.

The approach taken in the patch is that the keywords can be used with
any language version. If standard attributes were added in language
version Y, the keyword rules for version X<Y are the same as they were
for version Y (to the extent possible). Any extensions beyond Y are
handled in the same way for both keywords and attributes. This ensures
that existing C++11 successors like C++17 are not treated differently
from versions that have yet to be defined.

Some notes on the implementation:

* The patch emits errors rather than warnings for diagnostics that
relate to keywords.

* Where possible, the patch drops “attribute” from diagnostics
relating to keywords.

* One exception to the previous point is that warnings about C++
extensions do still mention attributes. The use there seemed OK
since the diagnostics are noting a change in the production rules.

* If a diagnostic string needs to be different for keywords and
attributes, the patch standardizes on passing the attribute/
name/token followed by 0 for attributes and 1 for keywords.

* Although the patch updates warn_attribute_wrong_decl_type_str,
warn_attribute_wrong_decl_type, and warn_attribute_wrong_decl_type,
only the error forms of these strings are used for keywords.

* I couldn't trigger the warnings in checkUnusedDeclAttributes,
even for existing attributes. An assert on the warnings caused
no failures in the testsuite. I think in practice all standard
attributes would be diagnosed before this.

* The patch drops a call to standardAttributesAllowed in
ParseFunctionDeclarator. This is because MaybeParseCXX11Attributes
checks the same thing itself, where appropriate.

* The new tests are based on c2x-attributes.c and
cxx0x-attributes.cpp. The C++ test also incorporates a version of
cxx11-base-spec-attributes.cpp. The FIXMEs are carried across from
the originals.

Differential Revision: https://reviews.llvm.org/D148702

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[clang][driver] Add -fplugin-arg- to pass arguments to plugins

From GCC's manpage:
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the
plugin called name.

[clang][driver] Add -fplugin-arg- to pass arguments to plugins

From GCC's manpage:
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the
plugin called name.

Since we don't have a key-value pair similar to gcc's plugin_argument
struct, simply accept key=value here anyway and pass it along as-is to
plugins.

This translates to the already existing '-plugin-arg-pluginname arg'
that clang cc1 accepts.

There is an ambiguity here because in clang, both the plugin name
as well as the option name can contain dashes, so when e.g. passing

-fplugin-arg-foo-bar-foo

it is not clear whether the plugin is foo-bar and the option is foo,
or the plugin is foo and the option is bar-foo. GCC solves this by
interpreting all dashes as part of the option name. So dashes can't be
part of the plugin name in this case.

Differential Revision: https://reviews.llvm.org/D113250

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Add a call super attribute plugin example

If a virtual method is marked as call_super, the
override method must call it, simpler feature like @CallSuper
in Android Java.