Implement P0482R2, support for char8_t type.

This is not yet part of any C++ working draft, and so is controlled by the flag
-fchar8_t rather than a -std= flag. (The GCC implementation is controlled by a
flag with the same name.)

This implementation is experimental, and will be removed or revised
substantially to match the proposal as it makes its way through the C++
committee.

llvm-svn: 331244
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index a1de22b..4b0220a 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -6841,6 +6841,9 @@
     case PREDEF_TYPE_NULLPTR_ID:
       T = Context.NullPtrTy;
       break;
+    case PREDEF_TYPE_CHAR8_ID:
+      T = Context.Char8Ty;
+      break;
     case PREDEF_TYPE_CHAR16_ID:
       T = Context.Char16Ty;
       break;