[IA] Recognize hexadecimal escape sequences

Summary:
Implement support for hexadecimal escape sequences to match how GNU 'as'
handles them. I.e., read all hexadecimal characters and truncate to the
lower 16 bits.

Reviewers: nickdesaulniers, jcai19

Subscribers: llvm-commits, hiraditya

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68598

llvm-svn: 374018