[clang] [CMake] Fix variable name (PR #127967)

[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` at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/18/builds/11773

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

```
Step 6 (test) failure: build (failure)
...
PASS: lldb-api :: tools/lldb-dap/instruction-breakpoint/TestDAP_instruction_breakpoint.py (1156 of 2908)
PASS: lldb-api :: tools/lldb-dap/locations/TestDAP_locations.py (1157 of 2908)
PASS: lldb-api :: tools/lldb-dap/memory/TestDAP_memory.py (1158 of 2908)
PASS: lldb-api :: tools/lldb-dap/module/TestDAP_module.py (1159 of 2908)
PASS: lldb-api :: tools/lldb-dap/evaluate/TestDAP_evaluate.py (1160 of 2908)
PASS: lldb-api :: tools/lldb-dap/optimized/TestDAP_optimized.py (1161 of 2908)
PASS: lldb-api :: tools/lldb-dap/output/TestDAP_output.py (1162 of 2908)
PASS: lldb-api :: tools/lldb-dap/repl-mode/TestDAP_repl_mode_detection.py (1163 of 2908)
UNSUPPORTED: lldb-api :: tools/lldb-dap/restart/TestDAP_restart_runInTerminal.py (1164 of 2908)
UNSUPPORTED: lldb-api :: tools/lldb-dap/runInTerminal/TestDAP_runInTerminal.py (1165 of 2908)
FAIL: lldb-api :: tools/lldb-dap/launch/TestDAP_launch.py (1166 of 2908)
******************** TEST 'lldb-api :: tools/lldb-dap/launch/TestDAP_launch.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 --arch armv8l --build-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex --lldb-module-cache-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex/module-cache-lldb/lldb-api --clang-module-cache-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex/module-cache-clang/lldb-api --executable /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin/lldb --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 /home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/test/API/tools/lldb-dap/launch -p TestDAP_launch.py
--
Exit Code: 1

Command Output (stdout):
--
lldb version 21.0.0git (https://github.com/llvm/llvm-project.git revision 81ed48531d51754fe4aac7a149ee4534a9ffda1b)
  clang revision 81ed48531d51754fe4aac7a149ee4534a9ffda1b
  llvm revision 81ed48531d51754fe4aac7a149ee4534a9ffda1b
Skipping the following test categories: ['libc++', 'dsym', 'gmodules', 'debugserver', 'objc']
========= DEBUG ADAPTER PROTOCOL LOGS =========
1740069970.014034510 --> 
Content-Length: 344

{
  "arguments": {
    "adapterID": "lldb-native",
    "clientID": "vscode",
    "columnsStartAt1": true,
    "linesStartAt1": true,
    "locale": "en-us",
    "pathFormat": "path",
    "sourceInitFile": false,
    "supportsRunInTerminalRequest": true,
    "supportsStartDebuggingRequest": true,
    "supportsVariablePaging": true,
    "supportsVariableType": true
  },
  "command": "initialize",
  "seq": 1,
  "type": "request"
}
1740069970.016421795 <-- 
Content-Length: 1631


```

</details>

https://github.com/llvm/llvm-project/pull/127967