[Assembler] Add location info to unary expressions.

Summary:
This is a continuation of D28861.  Add an SMLoc to MCUnaryExpr such that
a better diagnostic can be given in case of an error in later stages of
assembling.

Reviewers: rengolin, grosbach, javed.absar, olista01

Reviewed By: olista01

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30581

llvm-svn: 297454