ParseXcodeSDK: Register both the CU module and the SymbolFile module.

For Swift LLDB (but potentially also for module support in Clang-land)
we need a way to accumulate the path remappings produced by
Module::RegisterXcodeSDK(). In order to make this work for
SymbolFileDebugMaps, registering the search path remapping with both
modules is necessary.

Differential Revision: https://reviews.llvm.org/D79384

<rdar://problem/62750529>