[x86][inline-asm] Add support for curly brackets escape using "%" in extended inline asm.

Commit on behalf of mharoush

After LGTM and check all:

This patch is a compatibility fix for clang, matching GCC support for charter escape when using extended in-line assembly (i.e, "%{" ,"%}" --> "{" ,"}" ).
 It is meant to enable support for advanced features such as AVX512 conditional\masked vector instructions/broadcast assembly syntax.

Reviewer: 1. rnk

Differential Revision: https://reviews.llvm.org/D25012

llvm-svn: 285585