[llvm] [ADT] Tighten static_assert in Bitfields (PR #165099)

[LLVM Continuous Integration via llvm-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 `llvm` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/41902

<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/linker-flags.f90 (3944 of 3954)
PASS: Flang :: Transforms/debug-line-table.fir (3945 of 3954)
PASS: Flang :: Lower/OpenMP/rtl-flags.f90 (3946 of 3954)
PASS: Flang :: Driver/use-module-error.f90 (3947 of 3954)
PASS: Flang :: Transforms/tbaa3.fir (3948 of 3954)
PASS: Flang :: Transforms/debug-dwarf-version.fir (3949 of 3954)
PASS: Flang :: Transforms/tbaa-local-alloc-threshold.fir (3950 of 3954)
PASS: Flang :: Driver/fopenmp.f90 (3951 of 3954)
PASS: Flang :: Driver/omp-driver-offload.f90 (3952 of 3954)
PASS: Flang :: Intrinsics/math-codegen.fir (3953 of 3954)
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=3502.037892

```

</details>

https://github.com/llvm/llvm-project/pull/165099