Fix PCH detection in depend mode (+test improvements) (#427)

Fix PCH detection in depend mode (+test improvements) (#427)

* do not refer to Clang's PTH in tests

The PTH feature has been removed (https://reviews.llvm.org/D54547)
and according to the commit it has never really been used. Maybe this
made sense somewhen in the past, but now those .pth files must be PCHs
internally. This commit actually just changes the .pth extensions
to .pch to avoid confusion, technically nothing should change
except for filenames.

* try to share PCH tests between GCC and Clang

Clang is supposed to be a drop-in for GCC, so in general it should
be able to handle everything GCC can. That's not completely true
in practice, there are differences, but it doesn't make sense
to completely duplicate a testcase just because there are some
differences. So start creating a shared common base for the PCH
tests and do separately only tests that act differently.

* more sharing of PCH tests between GCC and Clang

There's e.g. no need to do all kinds of complex tests with both
.gch and .pch with Clang, except for checking that Clang finds
one of them if none is specified explicitly.

* log also when pch usage is detect from pragma pch_preprocess

* try harder to verify in tests that ccache detects PCH changes

Some of the tests did that, e.g. those 'file changed', but e.g. the cached
.gch creation did not. So try to intentionally change the .gch/.h and test
that it leads to a cache miss. Otherwise there might be a hit simply
because ccache failed to detect PCH usage and ignores the .gch completely.

* clean up #include vs -fpch-preprocess in pch tests

As the manpage says, -fpch-preprocess is needed only with the #include
form, otherwise it's pointless.

* do not mention sloppiness in pch tests, only no sloppiness

Since sloppiness is normally required, so no point in stating the obvious.

* test also -include-pch with clang

* hash also pch introduced only using -include

GCC does not output the pch in the .d dependencies file, so without
this there would be false cache hits.

* be consistent about sloppiness in pch tests

create pch -> pch_defines
use pch -> time_macros

* test CCACHE_PCH_EXTSUM more thoroughly and also with -include

* pch test for .gch file being in an extra directory

* doc corrections for how to use PCH with ccache

- ccache will fail to properly detect that -include a.h means using
  a.h.gch if it requires using path from -I (they are not searched)
- -fpch-preprocess does nothing with Clang, it doesn't output
  pragma GCC pch_preprocess and so #include form for PCHs doesn't work

* explain better problems of -MD/-MMD in depend mode