Move CompilerInstance::LLVMContext and LLVMContext ownership to CodeGenAction

This removes the final dependency edge from any lib outside of CodeGen
to core.  As a result we can, and do, trim the dependency on core
from libclang, PrintFunctionNames, the unit tests and c-index-test.
While at it, review and trim other unneeded dependencies.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@125820 91177308-0d34-0410-b5e6-96231b3b80d8