This patch is the fourth patch in a series of reviews for the Altmacro feature.
This patch introduces a new escape character '!' and it depends on D32701.
according to https://sourceware.org/binutils/docs/as/Altmacro.html:
"single-character string escape
To include any single character literally in a string (even if the character would otherwise have some special meaning), you can prefix the character with !' (an exclamation mark). For example, you can write <4.3 !> 5.4!!>' to get the literal text `4.3 > 5.4!'. "
Differential Revision: https://reviews.llvm.org/D32792
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302652 91177308-0d34-0410-b5e6-96231b3b80d8
