[Mips][msa] Add support for half

Add support for half (a.k.a. __fp16) in builtin descriptions. 
The second argument to BUILTIN() now accepts 'h' to represent half.

Patch by Daniel Sanders


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188464 91177308-0d34-0410-b5e6-96231b3b80d8