[Clang] Add a warning on invalid UTF-8 in comments.

Introduce an off-by default `-Winvalid-utf8` warning
that detects invalid UTF-8 code units sequences in comments.

Invalid UTF-8 in other places is already diagnosed,
as that cannot appear in identifiers and other grammar constructs.

The warning is off by default as its likely to be somewhat disruptive
otherwise.

This warning allows clang to conform to the yet-to be approved WG21
"P2295R5 Support for UTF-8 as a portable source file encoding"
paper.

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D128059