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.

This relands #167348.

The original PR was reverted due to a reported build failure, which was
later diagnosed as a local issue in the developer’s checkout or build
state. See discussion here:
https://github.com/llvm/llvm-project/pull/163659#discussion_r2511546964

No additional changes have been made in this reland.

Reverts #163659 due to missing one reference clang/Driver/Options.h in 
clang/include/clang/Driver/Driver.h.

See https://github.com/llvm/llvm-project/pull/163659#issuecomment-3512979187

This change moves option-related code from clangDriver into a new
clangOptions library.

This refactoring is part of a broader effort to support driver-managed
builds for compilations using C++ named modules and/or Clang modules.
It is required for linking the dependency scanning tooling against the
driver without introducing cyclic dependencies, which would otherwise
cause build failures when dynamic linking is enabled.
In particular, clangFrontend must no longer depend on clangDriver
for this to be possible.

This PR is motivated by the following review comment:
https://github.com/llvm/llvm-project/pull/152770#discussion_r2430756918

This PR passes the VFS down to `llvm::cl` functions so that they don't
assume the real file system.

This PR changes the behavior of `clang::ExecuteCompilerInvocation()` so
that it only returns early when the `DiagnosticsEngine` emitted errors
**within** the function. Handling errors emitted before the function got
called is a responsibility of the caller. Necessary for #158381.

This pr implements the functionality of `rootsig-define` as described
[here](https://github.com/llvm/wg-hlsl/blob/main/proposals/0029-root-signature-driver-options.md#option--rootsig-define).

This is accomplished by:
- Defining the `fdx-rootsignature-define`, and `rootsig-define` alias,
driver options. It simply specifies the name of a macro that will expand
to a `LiteralString` to be interpreted as a root signature.
- Introduces a new general frontend action wrapper,
`HLSLFrontendAction`. This class allows us to introduce `HLSL` specific
behaviour on the underlying action (primarily `ASTFrontendAction`).
Which will be further extended, or modularly wrapped, when considering
future DXC options.
- Using `HLSLFrontendAction` we can add a new `PPCallback` that will
eagerly parse the root signature specified with `rootsig-define` and
push it as a `TopLevelDecl` to `Sema`. This occurs when the macro has
been lexed.
- Since the root signature is parsed early, before any function
declarations, we can then simply attach it to the entry function once it
is encountered. Overwriting any applicable root signature attrs.

Resolves https://github.com/llvm/llvm-project/issues/150274

##### Implementation considerations

To implement this feature, note that:
1. We need access to all defined macros. These are created as part of
the first `Lex` in `Parser::Initialize` after `PP->EnterMainSourceFile`
2. `RootSignatureDecl` must be added to `Sema` before
`Consumer->HandleTranslationUnit` is invoked in `ParseAST`

Therefore, we can't handle the root signature in
`HLSLFrontendAction::ExecuteAction` before (from 1.) or after (from 2.)
invoking the underlying `ASTFrontendAction`.

This means we could alternatively:
- Manually handle this case
[here](https://github.com/llvm/llvm-project/blob/ac8f0bb070c9071742b6f6ce03bebc9d87217830/clang/lib/Parse/ParseAST.cpp#L168)
before parsing the first top level decl.
- Hook into when we [return the entry function
decl](https://github.com/llvm/llvm-project/blob/ac8f0bb070c9071742b6f6ce03bebc9d87217830/clang/lib/Parse/Parser.cpp#L1190)
and then parse the root signature and override its `RootSignatureAttr`.

The proposed solution handles this in the most modular way which should
work on any `FrontendAction` that might use the `Parser` without
invoking `ParseAST`, and, is not subject to needing to call the hook in
multiple different places of function declarators.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

This change introduces the cir-canonicalize pass. This is a simple
cir-to-cir transformation that eliminates empty scopes and redundant
branches. It will be expanded in future changes to simplify other
redundant instruction sequences.

MLIR verification and mlir-specific command-line option handling is also
introduced here.