Fix crash in libclang if code completion is used with unknown flags.

Use CaptureDroppedDiagnostics to make sure that there is a diagnostic
client installed when warning flags are parsed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186582 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Frontend/ASTUnit.cpp

@@ -2410,10 +2410,10 @@ void ASTUnit::CodeComplete(StringRef File, unsigned Line, unsigned Column,
     
   // Set up diagnostics, capturing any diagnostics produced.
   Clang->setDiagnostics(&Diag);
-  ProcessWarningOptions(Diag, CCInvocation->getDiagnosticOpts());
   CaptureDroppedDiagnostics Capture(true, 
                                     Clang->getDiagnostics(), 
                                     StoredDiagnostics);
+  ProcessWarningOptions(Diag, CCInvocation->getDiagnosticOpts());
   
   // Create the target instance.
   Clang->setTarget(TargetInfo::CreateTargetInfo(Clang->getDiagnostics(),

test/Index/c-index-unsupported-warning-test.c

@@ -0,0 +1,3 @@
+// RUN: c-index-test -code-completion-at=%s:1:1 -Wunknown-foo-bar-warning -Werror %s
+
+void f();