[analyzer][CTU] API for CTU macro expansions

Removes `CrossTranslationUnitContext::getImportedFromSourceLocation`
Removes the corresponding unit-test segment.

Introduces the `CrossTranslationUnitContext::getMacroExpansionContextForSourceLocation`
which will return the macro expansion context for an imported TU. Also adds a
few implementation FIXME notes where applicable, since this feature is
not implemented yet. This fact is also noted as Doxygen comments.

Uplifts a few CTU LIT test to match the current **incomplete** behavior.

It is a regression to some extent since now we don't expand any
macros in imported TUs. At least we don't crash anymore.

Note that the introduced function is already covered by LIT tests.
Eg.: Analysis/plist-macros-with-expansion-ctu.c

Reviewed By: balazske, Szelethus

Differential Revision: https://reviews.llvm.org/D94673