[mips] [IAS] Fix using .cpsetup with local labels (PR22518).

Summary:
Parse for an MCExpr instead of an Identifier and use the symbol for relocations, not just the symbol's name.

This fixes errors when using local labels in .cpsetup (PR22518).

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: seanbruno, emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D7697

llvm-svn: 229671