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

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.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D143657