[RISCV] Add +experimental-zvfh extension to cover half types in vectors.

Currently we allow half types in vectors if the scalar Zfh extension
is enabled. This behavior is not inline with the vector spec. For f32
and f64 types, the Zve32f, Zve64f, Zve64d, and V explicitly control
the availablity of floating point types in vectors.

In order to make our compiler compliant, we either need to remove all support
for half in vectors or we need an extension to control it.

Draft spec here https://github.com/riscv/riscv-v-spec/pull/780

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D121345