Fix a crash when ASTReader emits diagnostic when another one is in flight. Fixes rdar//9334563.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@130162 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/PCH/modified-header-crash.h b/test/PCH/modified-header-crash.h
new file mode 100644
index 0000000..971746e
--- /dev/null
+++ b/test/PCH/modified-header-crash.h
@@ -0,0 +1 @@
+int foo;