Fix use of config.h in public headers.

The CodeGenCoverage.h header is installed, but it references
the build-only header "llvm/Config/config.h". This breaks use
of the CodeGenCoverage.h header once it is installed, because config.h isn't
available.

This patch fixes the error by moving the config.h include from
the CodeGenCoverage.h header (where it's not needed), to the
CodeGenCoverage.cpp source file.

llvm-svn: 318602