[MC] Support symbolic expressions in assembly directives

This matches the behavior of GNU assembler which supports symbolic
expressions in absolute expressions used in assembly directives.

Differential Revision: http://reviews.llvm.org/D20337

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270786 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed