[Asm] Add debug printing for assembler macros

This adds some debug printing (gated behind the "asm-macros" debug flag) which
can help tracing complicated assembly macros.

Differential revision: https://reviews.llvm.org/D43937



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326795 91177308-0d34-0410-b5e6-96231b3b80d8