[clang] fix clang_cmake_builddir (PR #161220)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-linux-test-suite` running on `ppc64le-clang-test-suite` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/95/builds/18795

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

```
Step 6 (test-build-unified-tree-check-all) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
PASS: MemorySanitizer-POWERPC64LE :: allocator_padding.cpp (102770 of 102779)
PASS: Profile-powerpc64le :: Posix/instrprof-value-prof-shared.test (102771 of 102779)
PASS: lit :: shtest-define.py (102772 of 102779)
PASS: ThreadSanitizer-powerpc64le :: deadlock_detector_stress_test.cpp (102773 of 102779)
PASS: ThreadSanitizer-powerpc64le :: restore_stack.cpp (102774 of 102779)
PASS: SanitizerCommon-asan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (102775 of 102779)
PASS: SanitizerCommon-tsan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (102776 of 102779)
PASS: SanitizerCommon-ubsan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (102777 of 102779)
PASS: SanitizerCommon-msan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (102778 of 102779)
PASS: SanitizerCommon-lsan-powerpc64le-Linux :: Linux/signal_segv_handler.cpp (102779 of 102779)
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=4425.090010

```

</details>

https://github.com/llvm/llvm-project/pull/161220