Based on feedback from @Aaron.Ballman.
Remove the unused static ID char (can re-add it later if needed).
Add test to cover some invalid HLSL vector instantations ensuring
that the appropriate error messages are generated.
HLSL vector types are ext_vector types, but they are also exposed via a
template syntax `vector<T, #>`. This is morally equavalent to the code:
```c++
template <typename T, int Size>
using vector = T __attribute__((ext_vector_type(Size)))
```
The problem is that templates aren't supported before HLSL 2021, and
type aliases still aren't supported in HLSL.
To resolve this (and other issues where HLSL can't represent its own
types), we rely on an external AST & Sema source being registered for
HLSL code.
This patch adds the HLSLExternalSemaSource and registers the vector
type alias.
Depends on D127802
Differential Revision: https://reviews.llvm.org/D128012
Chris Bieneman