[CodeComplete] Fix crash when completing params function declarations.

[sammccall]

Summary

In a decl like int AA(BB cc) where BB isn't defined, we end up trying to
parse BB cc as an expression (vexing parse) and end up triggering the
parser's "recovery-in-function" completion with no actual function
scope.

This patch avoids the assumption that such a scope exists in this context.

[kadircet]

LGTM, thanks!

lib/Sema/SemaCodeComplete.cpp

if (Results.includeCodePatterns()) {
       Builder.AddChunk(CodeCompletionString::CK_RightParen);
       Builder.AddChunk(CodeCompletionString::CK_LeftBrace);
       Builder.AddChunk(CodeCompletionString::CK_VerticalSpace);
       Builder.AddChunk(CodeCompletionString::CK_RightBrace);
       Results.AddResult(Result(Builder.TakeString()));
     }
 
     // Switch-specific statements.
-    if (!SemaRef.getCurFunction()->SwitchStack.empty()) {
+    if (SemaRef.getCurFunction() &&
+        !SemaRef.getCurFunction()->SwitchStack.empty()) {
       // case expression:
       Builder.AddTypedTextChunk("case");
       Builder.AddChunk(CodeCompletionString::CK_HorizontalSpace);
       Builder.AddPlaceholderChunk("expression");
       Builder.AddChunk(CodeCompletionString::CK_Colon);
       Results.AddResult(Result(Builder.TakeString()));
 
       // default:

test/CodeCompletion/crash-func-decl.cpp

+// Important that BB is unknown.
+// This triggers completion in PCC_RecoveryInFunction context, with no function.
+int AA(BB cc);
+// RUN: not %clang_cc1 -fsyntax-only -code-completion-at=%s:3:12 %s | FileCheck %s
+// CHECK: COMPLETION: char