In one case, we have a null pointer check that's unnecessary because the
only caller of the function already asserts the value is non-null.

In the other case, we've got an anti-pattern of `is` followed by `get`.
The logic was easier to repair by changing `get` to `cast`.

Neither case is a functional change.

Fixes #145525

This pr provides the ability to specify the root signature version as a
compiler option and to retain this in the root signature decl.

It also updates the methods to serialize the version when dumping the
declaration and to output the version when generating the metadata.

- Update `DXContainer.hI` to define the root signature versions
- Update `Options.td` and `LangOpts.h` to define the
`fdx-rootsignature-version` compiler option
- Update `Options.td` to provide an alias `force-rootsig-ver` in
clang-dxc
- Update `Decl.[h|cpp]` and `SeamHLSL.cpp` so that `RootSignatureDecl`
will retain its version type
- Updates `CGHLSLRuntime.cpp` to generate the extra metadata field
- Add tests to illustrate

Resolves https://github.com/llvm/llvm-project/issues/126557.

Note: this does not implement validation based on versioning.
https://github.com/llvm/llvm-project/issues/129940 is required to
retrieve the version and use it for validations.

In #144957 the backend was updated to expect a version in the metadata,
but since the frontend wasn't updated this breaks compilation. This is a
somewhat temporary fix to that until #144813 lands.

BuiltIn variables were missing the visibility attribute, which caused
the Linkage capability to be emitted by the backend.

Implements

https://github.com/llvm/wg-hlsl/blob/main/proposals/0026-symbol-visibility.md.

The change is to stop using the `hlsl.export` attribute. Instead,
symbols with "program linkage" in HLSL will have export linkage with
default visibility, and symbols with "external linkage" in HLSL will
have export linkage with hidden visibility.

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.

The SPIR-V backend does not have access to the original name of a
resource in the source, so it tries to create a name. This leads to some
problems with reflection.
    
That is why start to pass the name of the resource from Clang to the
SPIR-V backend.
    
Fixes #138533

This commit is using the same mechanism as vk::ext_builtin_input to
implement the SV_Position semantic input.
The HLSL signature is not yet ready for DXIL, hence this commit only
implements the SPIR-V side.

This is incomplete as it doesn't allow the semantic on hull/domain and
other shaders, but it's a first step to validate the overall
input/output
semantic logic.

Fixes https://github.com/llvm/llvm-project/issues/136969

This variable attribute is used in HLSL to add Vulkan specific builtins
in a shader.
The attribute is documented here:

https://github.com/microsoft/hlsl-specs/blob/17727e88fd1cb09013cb3a144110826af05f4dd5/proposals/0011-inline-spirv.md

Those variable, even if marked as `static` are externally initialized by
the pipeline/driver/GPU. This is handled by moving them to a specific
address space `hlsl_input`, also added by this commit.

The design for input variables in Clang can be found here:
https://github.com/llvm/wg-hlsl/blob/355771361ef69259fef39a65caef8bff9cb4046d/proposals/0019-spirv-input-builtin.md


Co-authored-by: Justin Bogner <mail@justinbogner.com>

`HLSLRootSignature.h` was originally created to hold the struct
definitions of an `llvm::hlsl::rootsig::RootElement` and some helper
functions for it.

However, there many users of the structs that don't require any of the
helper methods. This requires us to link the `FrontendHLSL` library,
where we otherwise wouldn't need to.

For instance:
- This [revert](https://github.com/llvm/llvm-project/pull/142005) was
required as it requires linking to the unrequired `FrontendHLSL` library
- As part of the change required here:
https://github.com/llvm/llvm-project/issues/126557. We will want to add
an `HLSLRootSignatureVersion` enum. Ideally this could live with the
root signature struct defs, but we don't want to link the helper objects
into `clang/Basic/TargetOptions.h`

This change allows the struct definitions to be kept in a single header
file and to then have the `FrontendHLSL` library only be linked when
required.