Reland Implement _ExtInt as an extended int type specifier.

I fixed the LLDB issue, so re-applying the patch.

This reverts commit a4b88c044980337bb14390be654fe76864aa60ec.
diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index 825b062..466cb08 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -847,6 +847,7 @@
 
   case TST_typeofExpr:
   case TST_decltype:
+  case TST_extint:
     if (DS.getRepAsExpr() &&
         DS.getRepAsExpr()->containsUnexpandedParameterPack())
       return true;