[clang][ASTImporter] Add VaList declaration to lookup table.

The declaration of __builtin_va_list seems to be handled specially
by ASTContext and/or Sema. The normal AST traversal probably can
not find it, therefore it is not added to ASTImporterLookupTable.
If it is not added, errors can occur because a duplicated
VaList declaration is created at import. To fix the problem the
VaList declaration is added manually to ASTImporterLookupTable
at construction.
In special cases this declaration is inside a "invisible" 'std'
namespace that behaves the same way. This namespace must be added
to the table too.

Reviewed By: vabridgers, donat.nagy

Differential Revision: https://reviews.llvm.org/D144273