[HLSL][LongVec] Add Frexp long vector support

I predict this will be super easy once the intrinsic is implemented as it should just be adding `let VaryingLongVector = 1;` to `clang/include/clang/Basic/HLSLIntrinsics.td`

Start in clang/include/clang/Basic/HLSLIntrinsics.td and inspect how the Frexp intrinsic is represented for vector types. Add the requested long-vector support entry, then verify that Frexp accepts long vectors and that the relevant compiler checks pass.

Written by the indexing model from the issue text.