[clang] [LifetimeSafety] Use StringMap::contains (NFC) (PR #167186)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

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

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/42054

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

```
Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
PASS: Flang :: Driver/target-cpu-features.f90 (3968 of 3978)
PASS: Flang :: Driver/linker-flags.f90 (3969 of 3978)
PASS: Flang :: Intrinsics/math-codegen.fir (3970 of 3978)
PASS: Flang :: Driver/fveclib.f90 (3971 of 3978)
PASS: Flang :: Driver/use-module-error.f90 (3972 of 3978)
PASS: Flang :: Driver/linker-options.f90 (3973 of 3978)
PASS: Flang :: Driver/mcmodel.f90 (3974 of 3978)
PASS: Flang :: Driver/fopenmp.f90 (3975 of 3978)
PASS: Flang :: Driver/use-module.f90 (3976 of 3978)
PASS: Flang :: Driver/omp-driver-offload.f90 (3977 of 3978)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2485.982165

```

</details>

https://github.com/llvm/llvm-project/pull/167186