A '<' with a trigraph '#' is not a valid editor placeholder

Credit to OSS-Fuzz for discovery:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3137#c5

rdar://34923985

llvm-svn: 315398
diff --git a/clang/test/Parser/editor-placeholder-recovery.cpp b/clang/test/Parser/editor-placeholder-recovery.cpp
index 48c290e..d68e087 100644
--- a/clang/test/Parser/editor-placeholder-recovery.cpp
+++ b/clang/test/Parser/editor-placeholder-recovery.cpp
@@ -69,3 +69,7 @@
   // expected-error@-2 {{editor placeholder in source file}}
 #endif
 }
+
+void handleTrigraph() {
+  <??=placeholder#> // expected-error {{expected expression}} expected-error {{expected expression}} expected-warning {{trigraph converted to '#' character}}
+}