[Concepts] Concept Specialization Expressions

Part of C++20 Concepts implementation effort. Added Concept Specialization Expressions that are created when a concept is refe$

D41217 on Phabricator.

(recommit after fixing failing Parser test on windows)

llvm-svn: 374903
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 339f471..f2e6251 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -3403,6 +3403,7 @@
   case CXXUuidofExprClass:
   case OpaqueValueExprClass:
   case SourceLocExprClass:
+  case ConceptSpecializationExprClass:
     // These never have a side-effect.
     return false;