d076608d58d1ec55016eb747a995511e3a3f72aa moved some deps around to avoid
cycles and left clang/Frontend/FrontendDiagnostic.h as a shim that
simply includes clang/Basic/DiagnosticFrontend.h. This PR inlines it so
that nothing in tree still includes clang/Frontend/FrontendDiagnostic.h.

Doing this will help prevent future layering issues. See #162865.

Frontend already depends on Basic, so no new deps need to be added
anywhere except for places that do strict dep checking.

Since https://github.com/llvm/llvm-project/pull/158381 the
`CompilerInstance` is aware of the VFS and co-owns it. To reduce scope
of that PR, the VFS was being inherited from the `FileManager` during
`setFileManager()` if it wasn't configured before. However, the
implementation of that setter was buggy. This PR fixes the bug, and
moves us closer to the long-term goal of `CompilerInstance` requiring
the VFS to be configured explicitly and owned by the instance.

Update `.Cases` and `.CasesLower` with 4+ args to use the
`initializer_list` overload. The deprecation of these functions will
come in a separate PR.

For more context, see: https://github.com/llvm/llvm-project/pull/163405.

The `CompilerInstance::createSourceManager()` function currently accepts
the `FileManager` to be used. However, all clients call
`CompilerInstance::createFileManager()` prior to creating the
`SourceManager`, and it never makes sense to use a `FileManager` in the
`SourceManager` that's different from the rest of the compiler. Passing
the `FileManager` explicitly is redundant, error-prone, and deviates
from the style of other `CompilerInstance` initialization APIs.

This PR therefore removes the `FileManager` parameter from
`createSourceManager()` and also stops returning the `FileManager`
pointer from `createFileManager()`, since that was its primary use. Now,
`createSourceManager()` internally calls `getFileManager()` instead.

This PR makes the `VFS` parameter to `ASTUnit::LoadFromASTFile()`
required and explicit, rather than silently defaulting to the real file
system. This makes it easy to correctly propagate the fully-configured
VFS and load any input files like the rest of the compiler does.

This PR is a part of the effort to make the VFS used in the compiler
more explicit and consistent.

Instead of creating the VFS deep within the compiler (in
`CompilerInstance::createFileManager()`), clients are now required to
explicitly call `CompilerInstance::createVirtualFileSystem()` and
provide the base VFS from the outside.

This PR also helps in breaking up the dependency cycle where creating a
properly configured `DiagnosticsEngine` requires a properly configured
VFS, but creating properly configuring a VFS requires the
`DiagnosticsEngine`.

Both `CompilerInstance::create{FileManager,Diagnostics}()` now just use
the VFS already in `CompilerInstance` instead of taking one as a
parameter, making the VFS consistent across the instance sub-object.

Clang modules sort the umbrella dir headers by name before adding to the
module's includes to ensure deterministic output across different file
systems.
This is insufficient however, as the header search table is also
serialized.
This includes all the loaded headers by file reference, which are
allocated
incrementally. To ensure stable output we have to also create the file
references in sorted order.

Thanks to Justin Stitt for bringing this up, providing test cases and a
direction for the fix!

This commit handles the following types:
- clang::ExternalASTSource
- clang::TargetInfo
- clang::ASTContext
- clang::SourceManager
- clang::FileManager

Part of cleanup #151026

Unlike other declarations, these cover two ranges:
- from `namespace/inline namespace` to the opening `{`,
- the closing `}`.

This allows to mark the declarations inside the namespace itself
independently.