[ASTUnit] Clear out diagnostic state after creating the preamble.

If the preamble had diagnostic state this would leave behind invalid
state in the DiagnosticsEngine and crash later. The test case runs into
an assertion in DiagnosticsEngine::setSourceManager.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294963 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Frontend/ASTUnit.cpp

@@ -1891,6 +1891,8 @@ bool ASTUnit::LoadFromCompilerInvocation(
     PreambleRebuildCounter = PrecompilePreambleAfterNParses;
     OverrideMainBuffer =
         getMainBufferWithPrecompiledPreamble(PCHContainerOps, *Invocation);
+    getDiagnostics().Reset();
+    ProcessWarningOptions(getDiagnostics(), Invocation->getDiagnosticOpts());
   }
   
   SimpleTimer ParsingTimer(WantTiming);

test/Index/complete-preamble.h

@@ -1,6 +1,11 @@
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Weverything"
+
 namespace std {
   void wibble();
 }
 
+#pragma clang diagnostic pop
+
 namespace std {
 }