[clang] [clang-tools-extra] [clangd][clang-tidy] Factor out mergeFixits() logic and reuse it in clangd (PR #208682)

[Zeyi Xu via cfe-commits]

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@@ -2227,6 +2204,36 @@ TEST(DiagnosticsTest, DontSuppressSubcategories) {
               ElementsAre(diagName("-Wunreachable-code-break")));
 }
 
+TEST(DiagnosticsTest, TokenMergeFixit) {
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zeyi2 wrote:

I think `readability-redundant-parentheses` already produces the correct fix-it after #202365, so we may need a different way to test this patch. That could be a bit tricky, though.

https://github.com/llvm/llvm-project/pull/208682