[Concepts] Requires Expressions

Implement support for C++2a requires-expressions.

Re-commit after compilation failure on some platforms due to alignment issues with PointerIntPair.

Differential Revision: https://reviews.llvm.org/D50360
diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 5aac7a8..60f1dba 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -111,6 +111,7 @@
   case Decl::Empty:
   case Decl::Concept:
   case Decl::LifetimeExtendedTemporary:
+  case Decl::RequiresExprBody:
     // None of these decls require codegen support.
     return;