[AsmParser] make .ascii support spaces as separators

Currently the integrated assembler only allows commas as the separator
between string arguments in .ascii. This patch adds support to using
space as separators and make IAS consistent with GNU assembler.

Link: https://github.com/ClangBuiltLinux/linux/issues/1196

Reviewed By: nickdesaulniers, jrtc27

Differential Revision: https://reviews.llvm.org/D91460