[AsmParser] Recognize more escaped characters between single quotes

The GNU AS manual states the following about single-character constants enclosed within single quotes:

>  Some backslash escapes apply to characters, \b, \f, \n, \r, \t, and \" with the same meaning as for strings, plus \' for a single quote.

Add two more characters to the switch handling this case to match GAS behaviour, plus a test to make sure nothing regresses.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D99609