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

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `openmp-s390x-linux` running on `systemz-1` while building `clang-tools-extra,clang` at step 6 "test-openmp".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/88/builds/25757

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-openmp) failure: 1200 seconds without output running [b'ninja', b'-j 4', b'check-openmp'], attempting to kill
...
PASS: libomp :: worksharing/sections/omp_section_lastprivate.c (423 of 434)
PASS: libomp :: worksharing/sections/omp_section_private.c (424 of 434)
PASS: libomp :: worksharing/sections/omp_sections_nowait.c (425 of 434)
PASS: libomp :: worksharing/sections/omp_sections_reduction.c (426 of 434)
PASS: libomp :: worksharing/single/omp_single_nowait.c (427 of 434)
PASS: libomp :: worksharing/single/omp_single_private.c (428 of 434)
PASS: libomp :: worksharing/for/omp_for_collapse.c (429 of 434)
PASS: libomp :: worksharing/for/omp_for_collapse_LowerTriangularLess.c (430 of 434)
PASS: libomp :: worksharing/single/omp_single.c (431 of 434)
PASS: libomp :: worksharing/single/omp_single_copyprivate.c (432 of 434)
command timed out: 1200 seconds without output running [b'ninja', b'-j 4', b'check-openmp'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1379.578708

```

</details>

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