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

[Peiqi Li via cfe-commits]

================
@@ -166,6 +166,7 @@ clang_target_link_libraries(clangDaemon
   clangBasic
   clangDependencyScanning
   clangDriver
+  clangEdit
----------------
voyager-jhk wrote:

I measured the clangd binary size before and after this change.

Before:
text  data  bss
70250470 1928512 279040

After:
text  data  bss
70251462 1928480 279040

So the text size increased by about 1KB.
This seems to be within the expected range for this change.

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