Fix the TestCppNsImport test case for DWARF. It was failing for DWARF in .o...

Fix the TestCppNsImport test case for DWARF. It was failing for DWARF in .o files because SymbolFileDWARFDebugMap didn't implement the following function:

void
ParseDeclsForContext (lldb_private::CompilerDeclContext decl_ctx) override;
                   
Now it does and the test works.

llvm-svn: 249626