[AST][NFC] Various NFCs in StringLiteral

Factored out of D54166
([AST] Store the string data in StringLiteral in a trailing array of chars):

* For-range loops in containsNonAscii and containsNonAsciiOrNull.
* Comments and style fixes.
* int -> unsigned in mapCharByteWidth since TargetInfo::getCharWidth
  and friends return an unsigned, and StringLiteral manipulates and
  stores CharByteWidth as an unsigned.

llvm-svn: 346967