[lldb] Introduce SymbolFile::ParseAllLanguages

SymbolFile::ParseAllLanguages allows collecting the languages of the
extra compile units a SymbolFileDWARFDebugMap may have, which can't
be accessed otherwise. For every other symbol file type, it should
behave exactly the same as ParseLanguage.

rdar://97610458

Differential Revision: https://reviews.llvm.org/D146265