[clang] [clang][bytecode] Check source pointer for bitcast validity (PR #166907)

[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/42076

<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 :: Transforms/tbaa-local-alloc-threshold.fir (3969 of 3979)
PASS: Flang :: Transforms/stack-arrays.f90 (3970 of 3979)
PASS: Flang :: Transforms/stack-arrays-hlfir.f90 (3971 of 3979)
PASS: Flang :: Transforms/simplify-fir-operations.fir (3972 of 3979)
PASS: Flang :: Transforms/debug-line-table.fir (3973 of 3979)
PASS: Flang :: Transforms/debug-dwarf-version.fir (3974 of 3979)
PASS: Flang :: Transforms/OpenACC/acc-implicit-data-fortran.F90 (3975 of 3979)
PASS: Flang :: Driver/omp-driver-offload.f90 (3976 of 3979)
PASS: Flang :: Semantics/modfile03.f90 (3977 of 3979)
PASS: Flang :: Intrinsics/math-codegen.fir (3978 of 3979)
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=2233.290325

```

</details>

https://github.com/llvm/llvm-project/pull/166907