[SymbolFile] Implement GetCompleteObjCClass for .debug_names

When running the test suite with .debug_names a bunch of tests were
failing because GetCompleteObjCClass was not yet implemented for
DebugNamesDWARFIndex. This patch adds the required logic.

We use the .debug_names to find the Objective-C class and then rely on
DW_AT_APPLE_objc_complete_type to find the complete type. If we can't
find it or the attribute is not supported, we return a list of potential
complete types.

Differential revision: https://reviews.llvm.org/D48596

llvm-svn: 335776