remove gcc::Precompile

Clang still has support for running gcc for performing various stages
of a build. Right now it looks like this is used for

* Supporting Fortran in the clang driver
* Running an assembler or linker in systems we don't yet know how to
run them directly.

It looks like the gcc::Precompile is a vestige from the days when we
supported using clang for C and running gcc for c++. This patch removes it
(yes, we have no tests for it).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195586 91177308-0d34-0410-b5e6-96231b3b80d8