[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 `lldb-arm-ubuntu` running on `linaro-lldb-arm-ubuntu` while building `clang-tools-extra,clang` at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/18/builds/26459

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

```
Step 6 (test) failure: build (failure)
...
PASS: lldb-unit :: tools/lldb-mcp/./LLDBMCPTests/9/23 (2987 of 2996)
PASS: lldb-unit :: tools/lldb-server/tests/./LLDBServerTests/0/3 (2988 of 2996)
PASS: lldb-unit :: tools/lldb-server/tests/./LLDBServerTests/1/3 (2989 of 2996)
PASS: lldb-unit :: tools/lldb-server/tests/./LLDBServerTests/2/3 (2990 of 2996)
PASS: lldb-unit :: Process/gdb-remote/./ProcessGdbRemoteTests/7/22 (2991 of 2996)
PASS: lldb-unit :: Process/gdb-remote/./ProcessGdbRemoteTests/8/22 (2992 of 2996)
PASS: lldb-unit :: Utility/./UtilityTests/21/26 (2993 of 2996)
PASS: lldb-unit :: Utility/./UtilityTests/22/26 (2994 of 2996)
PASS: lldb-api :: tools/lldb-dap/coreFile/TestDAP_coreFile.py (2995 of 2996)
TIMEOUT: lldb-api :: tools/lldb-dap/databreakpoint/TestDAP_setDataBreakpoints.py (2996 of 2996)
******************** TEST 'lldb-api :: tools/lldb-dap/databreakpoint/TestDAP_setDataBreakpoints.py' FAILED ********************
Script:
--
/usr/bin/python3.10 /home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/test/API/dotest.py -u CXXFLAGS -u CFLAGS --env LLVM_LIBS_DIR=/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./lib --env LLVM_INCLUDE_DIR=/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/include --env LLVM_TOOLS_DIR=/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin --triple armv8l-unknown-linux-gnueabihf --build-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build --lldb-module-cache-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build/module-cache-lldb/lldb-api --clang-module-cache-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build/module-cache-clang/lldb-api --executable /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin/lldb --lldb-python-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/local/lib/python3.10/dist-packages --compiler /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin/clang --dsymutil /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin/dsymutil --make /usr/bin/gmake --llvm-tools-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin --lldb-obj-root /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/tools/lldb --lldb-libs-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./lib --cmake-build-type Release /home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/test/API/tools/lldb-dap/databreakpoint -p TestDAP_setDataBreakpoints.py --timeout 600
--
Exit Code: -9
Timeout: Reached timeout of 600 seconds

Command Output (stdout):
--
Skipping the following test categories: libc++, msvcstl, dsym, pdb, gmodules, debugserver, objc

--
Command Output (stderr):
--
PASS: LLDB (/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/bin/clang-arm) :: test_bytes (TestDAP_setDataBreakpoints.TestDAP_setDataBreakpoints)
PASS: LLDB (/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/bin/clang-arm) :: test_duplicate_start_addresses (TestDAP_setDataBreakpoints.TestDAP_setDataBreakpoints)
FAIL: LLDB (/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/bin/clang-arm) :: test_expression (TestDAP_setDataBreakpoints.TestDAP_setDataBreakpoints)
Log Files:
 - /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build/tools/lldb-dap/databreakpoint/TestDAP_setDataBreakpoints/Error_test_expression.log
PASS: LLDB (/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/bin/clang-arm) :: test_functionality (TestDAP_setDataBreakpoints.TestDAP_setDataBreakpoints)
======================================================================
ERROR: test_expression (TestDAP_setDataBreakpoints.TestDAP_setDataBreakpoints)
   Tests setting data breakpoints on expression.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 1211, in tearDown
    hook()  # try the plain call and hope it works
  File "/home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/packages/Python/lldbsuite/test/tools/lldb_dap/testcase.py", line 114, in cleanup_session
    session.disconnect(terminateDebuggee=True)
  File "/home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/packages/Python/lldbsuite/test/tools/lldb_dap/session_helpers.py", line 1718, in disconnect
    response = self.send_request(args).result()
  File "/home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/packages/Python/lldbsuite/test/tools/lldb_dap/session.py", line 99, in result
    response = self.result_or_error()
  File "/home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/packages/Python/lldbsuite/test/tools/lldb_dap/session.py", line 116, in result_or_error
    raw = self._future.result(timeout=self._timeout)
  File "/usr/lib/python3.10/concurrent/futures/_base.py", line 460, in result
    raise TimeoutError()
concurrent.futures._base.TimeoutError: 

```

</details>

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