Add an explicit API to read the Xcode SDK DWARF attribute from compile units

When debugging from a SymbolMap the creation of CompileUnits for the
individual object files is so lazy that RegisterXcodeSDK() is not
invoked at all before the Swift TypeSystem wants to read it. This
patch fixes this by introducing an explicit
SymbolFile::ParseXcodeSDK() call that can be invoked deterministically
before the result is required.

<rdar://problem/62532151+62326862>

https://reviews.llvm.org/D79273