[CodeGenPrepare] Move CodeGenPrepare into lib/CodeGen.

CodeGenPrepare uses extensively TargetLowering which is part of libLLVMCodeGen.
This is a layer violation which would introduce eventually a dependence on
CodeGen in ScalarOpts.

Move CodeGenPrepare into libLLVMCodeGen to avoid that.

Follow-up of <rdar://problem/15519855>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201912 91177308-0d34-0410-b5e6-96231b3b80d8
Switch the LLVM error reporting on the new diagnostic system.
The LLVM diagnostic are now wired-up in clang (since r200931),
thus the user experience will not be impacted by this change
anymore.

Related to <rdar://problem/15886697>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201915 91177308-0d34-0410-b5e6-96231b3b80d8
Use relative rpath so that the installation and build dirs are relocatable.

This works by asking cmake to use the "install rpath", but setting that rpath
to be relative.

Thanks a lot to Brad King for the help with CMake!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201921 91177308-0d34-0410-b5e6-96231b3b80d8