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

[Peiqi Li 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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voyager-jhk wrote:

I've update it----it now injects a synthetic diagnostic with naive removal fix-its into StoreDiags. PTAL!

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