Switch the module-loading interfaces and parser from a simple

top-level module name to a module path (e.g., std.vector). We're still
missing a number of pieces for this actually to do something.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145462 91177308-0d34-0410-b5e6-96231b3b80d8