[VectorUtils] Expose vector-function-abi-variant mangling as a utility.

Summary:
This change exposes the vector name mangling with LLVM ISA (used as part
of vector-function-abi-variant) as a utility.
This can then be used by front-ends that add this attribute.
Note that all parameters passed in to the function will be mangled with
the "v" token to identify that they are of of vector type. So, it is the
responsibility of the caller to confirm that all parameters in the
vectorized variant is of vector type.

Added unit test to show vector name mangling.

Reviewed-By: fpetrogalli, simoll

Differential Revision: https://reviews.llvm.org/D79867