Deleted almost all of the precomp.cpp files.

Looks like we don't actually use them; when creating precompiled headers
with Visual C++, a dummy .cpp is created at build time.
Made it so that a dummy .cpp is actually created when compiling with VC++.
Renamed ocl tests' precomp files according to convention.

Also, added the missing include guard to perf_precomp.hpp.
This should fix the build.
Merge pull request #1241 from SpecLad:redundant-precomps