[clang-diff] Fix assertion error when dealing with wide strings

Directly using StringLiteral::getString for wide string is not
currently supported; therefore in ASTDiff, getStmtValue will fail when
asserting that the StringLiteral has a width of 1. This patch also
covers cases for UTF16 and UTF32 encoding, along with corresponding
test cases.

Fixes https://github.com/llvm/llvm-project/issues/55771.

Reviewed By: johannes

Differential Revision: https://reviews.llvm.org/D126651