[AST] Fix warnings

This patch fixes:

  clang/lib/AST/Type.cpp:4893:80: error: multi-line // comment
  [-Werror,-Wcomment]

  clang/lib/AST/Type.cpp:4894:80: error: multi-line // comment
  [-Werror,-Wcomment]

  clang/lib/AST/Type.cpp:4891:11: error: 12 enumeration values not
  handled in switch: 'Adjusted', 'Decayed',
  'Attributed'... [-Werror,-Wswitch]

Author: kazutakahirata

clang/lib/AST/Type.cpp

@@ -4890,7 +4890,7 @@ bool Type::canHaveNullability(bool ResultIfUnknown) const {
 
   switch (type->getTypeClass()) {
 #define NON_CANONICAL_TYPE(Class, Parent)                                      \
-  // We'll only see canonical types here.                                      \
+  /* We'll only see canonical types here. */                                   \
   case Type::Class:                                                            \
     llvm_unreachable("non-canonical type");
 #define TYPE(Class, Parent)