Fix layering of file remapping and header search initialization

These two functions initialize the source manager and header search objects and
shouldn't be in InitPreprocessor which is concerned with priming the
preprocessor itself and predefining macros.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212434 91177308-0d34-0410-b5e6-96231b3b80d8