[Clang][RISCV] Guard vector float16 type correctly with semantic analysis

[eopXD]

Summary

Before this commit, vector float 16 types (e.g. vfloat16m1_t) of RVV
is only defined when extension zvfh is defined. However this
generate inaccurate diagnostics like:

error: unknown type name 'vfloat16m1_t'

This commit improves the compiler by guarding type check correctly
under semantic analysis.

[craig.topper]

Are you going to do f32, f64, and i64 as well?

[eopXD]

In D143657#4115721, @craig.topper wrote:

Are you going to do f32, f64, and i64 as well?

Yes, going to add them once this commit lands.

[eopXD]

Forward declaration (snippet below) is still allowed after this commit.

#include <riscv_vector.h>

vfloat16m1_t foo();

I see the same behavior under https://github.com/llvm/llvm-project/blob/main/clang/test/Sema/x86_64-no-x87.cpp. If I add type check under SemaDecl.cpp::Sema::ActOnFunctionDeclarator, the compiler will emit repeating warning for the same function. Currently I don't know what is the best fix for this.

[eopXD]

Update code: rename error for vector support, remove dead code under SemaRISCVVectorLoopup.cpp

[eopXD]

Update code: check for experimental-zvfh instead of zvfh.

[eopXD]

Update code based on comments from Craig.

[eopXD]

Update code: use a more scalable way to check for RVV type.

[craig.topper]

clang/lib/Sema/Sema.cpp

2045

experimental- is an internal naming scheme. "experimental-" is not used in -march so it should be in the error message.

[eopXD]

Address comment from Craig.

[craig.topper]

LGTM