Skip Unicode character expansion in assembly files

Summary: When using the C preprocessor with assembly files, either with a
capital `S` file extension, or with `-xassembler-with-cpp`, the Unicode escape
sequence `\u` is ignored. The `\u` pattern can be used for expanding a macro
argument that starts with `u`.

Author: Salman Arif <salman.arif@arm.com>

Reviewers: rengolin, olista01

Reviewed By: olista01

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31765


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299754 91177308-0d34-0410-b5e6-96231b3b80d8