[clang] [CIR] Upstream initial support for fixed size VectorType (PR #136488)

[Andy Kaylor via cfe-commits]

================
@@ -82,6 +82,9 @@ class CIRGenBuilderTy : public cir::CIRBaseBuilderTy {
                   cir::IntType>(ty))
       return true;
 
+    if (mlir::isa<cir::VectorType>(ty))
----------------
andykaylor wrote:

```suggestion
    if (auto vt = mlir::dyn_cast<cir::VectorType>(ty))
```
Then this would replace the `mlir::cast` in the statement below.

https://github.com/llvm/llvm-project/pull/136488