[clang] ASTImporter: Fix importing of va_list types and declarations

This fixes a problem where __va_list_tag was not correctly imported,
possibly leading to multiple definitions with different types.

This adds __va_list_tag to it's proper scope, so that the ASTImporter
can find it.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Differential Revision: https://reviews.llvm.org/D136886