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.

llvm-svn: 125820