Log Message:
include-what-you-use: updated to 0.12

iwyu 0.12 compatible with llvm+clang 8.0 is released. Major changes:
[iwyu] New command-line option: --keep to mirror IWYU pragma: keep
[iwyu] New command-line option: --cxx17ns to suggest compact C++17 nested namespaces
[iwyu] Improve --no_fwd_decls to optimize for minimal number of redeclarations
[iwyu] Improved mappings for POSIX types and let mappings apply to builtins as well
[iwyu] More principled handling of explicit template instantiations
[iwyu_tool] Breaking change: extra args are no longer automatically prefixed \ 
with -Xiwyu (so you can use them for Clang args too)
[iwyu_tool] Better Windows support
[fix_includes] Better handling of template forward-decls