Support: Fix incremental build when re-configuring targets

r180893 added an indirect include of llvm/Config/Targets.def to
llvm/Support/CodeGen.h, which in turn is included by things like
llvm/IR/Module.h.  After a full build of LLVM and Clang, ninja had to
rebuild 1274 files after reconfiguring.

This commit strips CodeGen.h back down to just a pile of enums and moves
the expensive includes over to CodeGenCWrappers.h (which is only
included in two places).  This gets ninja down to 88 files if you
reconfigure with, e.g., -DLLVM_TARGETS_TO_BUILD=X86.

llvm-svn: 260835