IWYU 0.19

Compatible with Clang 15.

• [iwyu] New --comment_style option to control verbosity of 'why' comments
• [iwyu] New --regex option to select regex dialect
• [iwyu] Add support for regex replacement in mappings
• [iwyu] Add begin_keep/end_keep pragmas for protecting ranges of includes or forward-declares
• [iwyu] Fix several crasher bugs for unusual inputs
• [iwyu] More exhaustive handling of type aliases and enums
• [iwyu] Recognize IWYU pragmas in CRLF source files
• [iwyu] Respect configured toolchain on macOS (and overrides via -nostdinc++ + -isystem)
• [fix_includes] Recognize namespace alias declarations
• [mappings] Improve mappings for POSIX and libc headers
• [cmake] Build now requires a C++17 compiler (as does LLVM)
• [cmake] Support LLVM external project build (see README)

... and many other changes