[HLSL] Move `Sema` work out of `Parser::ParseMicrosoftRootSignatureAttributeArgs`

[inbelic]

It was brought up here, that the initial implementation was over-zealous in doing some Sema work in the Parser library. The Sema work is described below.

For context, when we parse and create RootSignatureDecl, we want to allow ourselves to only parse identical root signatures once. To do so, when we create a new RootSignatureDecl, we will place this implicit decl on the current scope. Then before we invoke the parser we will check to see if a corresponding decl was already created and instead just point to that decl.

So then, the Sema work being done is:

• the storing of the Decl onto the current scope
• the subsequent look-ups from the scope
• creation of the actual declaration

This issue tracks to move the above Sema work to be in Sema.

Proposed Solution:

Create two new functions ActOnStartRootSignatureDecl and ActOnFinishRootSignatureDecl.

The former will contain the creation of the unique DeclIdent from the root signature string and then try to lookup if the Decl has been previously parsed. It will return the DeclIdent and a bool if it was found.

The latter will contain the logic to take the parsed root elements (if parsed) and construct a RootSignatureDecl and then put this onto the scope with PushOnScopeChains.

The updated flow would look something like:

auto [DeclIdent, Found] = Actions.ActOnStartRootSignatureDecl(Signature);
if (!Found) {
  // Invoke RootSignatureParser
  ...
  Actions.ActOnFinishRootSignatureDecl(RootElements);
}
// Create ParsedArg
...

It is currently defined here:

llvm-project/clang/lib/Parse/ParseDeclCXX.cpp

Line 4943 in 599b2a3

// Construct our identifier

.

AC:

• Any Sema logic currently contained in Parser::ParseMicrosoftRootSignatureAttributeArgs should be moved out and encapsulated with Sema
• From the perspective of test-cases, this is expected to be a nfc

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Finn Plummer (inbelic)

It was brought up [here](https://github.com//pull/137690#discussion_r2109967008), that the initial implementation was over-zealous in doing some `Sema` work in the `Parser` library. The `Sema` work is described below.

For context, when we parse and create RootSignatureDecl, we want to allow ourselves to only parse identical root signatures once. To do so, when we create a new RootSignatureDecl, we will place this implicit decl on the current scope. Then before we invoke the parser we will check to see if a corresponding decl was already created and instead just point to that decl.

So then, the Sema work being done is:

• the storing of the Decl onto the current scope
• the subsequent look-ups from the scope
• creation of the actual declaration

This issue tracks to move the above Sema work to be in Sema.

Proposed Solution:

Create two new functions ActOnStartRootSignatureDecl and ActOnFinishRootSignatureDecl.

The former will contain the creation of the unique DeclIdent from the root signature string and then try to lookup if the Decl has been previously parsed. It will return the DeclIdent and a bool if it was found.

The latter will contain the logic to take the parsed root elements (if parsed) and construct a RootSignatureDecl and then put this onto the scope with PushOnScopeChains.

The updated flow would look something like:

auto [DeclIdent, Found] = Actions.ActOnStartRootSignatureDecl(Signature);
if (!Found) {
  // Invoke RootSignatureParser
  ...
  Actions.ActOnFinishRootSignatureDecl(RootElements);
}
// Create ParsedArg
...

It is currently defined here:

llvm-project/clang/lib/Parse/ParseDeclCXX.cpp

Line 4943 in 599b2a3

// Construct our identifier

.

AC:

• Any Sema logic currently contained in Parser::ParseMicrosoftRootSignatureAttributeArgs should be moved out and encapsulated with Sema
• From the perspective of test-cases, this is expected to be a nfc