For `c-header` and `c++-header` inputs, Clang driver selects the
`-emit-pch` frontend action and defaults the output suffix name to
`.gch` (except clang-cl, which uses `.pch`) to follow GCC.
```
clang -c a.h # a.h.gch
clang -xc++-header -c a.h # a.h.gch
```
In every other place we prefer `.pch` to `.gch`. E.g. `-include a.h`
probes `a.h.pch` before `a.h.gch`, and we have `-include-pch` instead of
`-include-gch`. (Clang and GCC use different extension names for C++
modules as well.)
This change isn't disruptive because:
* Most newer build systems specify explicit output filenames and are
unaffected
* Implicit output name users almost assuredly use something like
`-include a.h`, which probes both `a.h.pch` and `a.h.gch`[^1]
[^1]: In the future, we shall stop probing `a.h.gch` for `-include a.h`
as the
compatibility intended behavior actually gets in the way
(https://discourse.llvm.org/t/how-to-have-clang-ignore-gch-directories/51835).
This patch is a prerequisite.
Using -fmodules-* options for PCHs is a bit confusing, so add -fpch-*
variants. Having extra options also makes it simple to do a configure
check for the feature.
Also document the options in the release notes.
Differential Revision: https://reviews.llvm.org/D83623
This way should be the same like with a.pcm for modules.
An alternative way is 'clang++ -c empty.cpp -include-pch a.pch -o a.o
-Xclang -building-pch-with-obj', which is what clang-cl's /Yc does
internally.
Differential Revision: https://reviews.llvm.org/D83716
