[clang] [clang-format] Don't crash on incomplete template declaration (PR #173433)

[via cfe-commits]

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@@ -29053,6 +29053,8 @@ TEST_F(FormatTest, KeywordedFunctionLikeMacros) {
                Style);
 }
 
+TEST_F(FormatTest, DoesNotCrashIncompleteCode) { verifyNoCrash("template <"); }
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owenca wrote:

```suggestion
TEST_F(FormatTest, UnbalancedAngleBrackets) { verifyFormat("template <"); }
```
We can put this test case and the one from #173456 in the same `TEST_F`. Also, it's better to use `verifyFormat` for incomplete code as some editors support formatting as you type.

https://github.com/llvm/llvm-project/pull/173433