[AST] Use correct APSInt width when evaluating string literals

The width of the APSInt values should be the width of an element.
getCharByteWidth returns the size of an element in _host_ bytes, which
makes the width N times greater, where N is the ratio between target's
CHAR_BIT and host's CHAR_BIT.
This is NFC for in-tree targets because all of them have CHAR_BIT == 8.

Reviewed By: cor3ntin, aaron.ballman

Differential Revision: https://reviews.llvm.org/D154773