RFC - Remove support for PCH post 4.8

[Diego Novillo]

I admit that I'm partly fishing here, but my proposal is based on the following:

* The implementation of PCH in GCC is atrocious and hard to maintain.
* The next C++ standard is likely to define modules
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3347.pdf)
* The user-base for PCH is negligibly small (here's where I'm mostly guessing).
* PCH is absolutely useless for C.

Removing PCH will give us more implementation freedom for the memory
management project
(http://gcc.gnu.org/wiki/cxx-conversion/gc-alternatives).

With some effort, we could revive the streaming work done in the PPH
branch and re-implement PCH with it (when we abandoned the branch, we
were probably 80% complete as a PCH replacement). If we could at least
remove support for C, then I can see a streaming-based PCH for the 4.9
release.

Thoughts?

Are there any big PCH users out there?


Thanks.  Diego.