Move some functions from DWARFASTParserClang to ClangASTImporter.

This allows these functions to be re-used by a forthcoming
PDBASTParser. The functions in question are CanCompleteType,
CompleteType, and CanImport. Conceptually, these functions belong
on ClangASTImporter anyway, and previously they were just ping
ponging around through a few levels of indirection to end up there
as well, so this patch actually makes the code somewhat simpler.

A few methods were moved to a new file called ClangUtil, so that
they can be shared between ClangASTImporter and ClangASTContext
without creating a circular dependency between those two cpp
files.

Differential Revision: http://reviews.llvm.org/D18381