[Assembler] Improve error when unable to evaluate expression.

Add a SMLoc to MCExpr. Most code does not generate or consume the SMLoc (yet).

Patch by Sanne Wouda <sanne.wouda@arm.com>!
Differential Revision: https://reviews.llvm.org/D28861

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292515 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed