[clang] clang/Driver: Use struct type for BoundArch instead of StringRef (PR #204748)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-x86_64-debian-dylib` running on `gribozavr4` while building `clang` at step 9 "test-build-unified-tree-check-clang-extra".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/60/builds/2475

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

```
Step 9 (test-build-unified-tree-check-clang-extra) failure: 1200 seconds without output running [b'ninja', b'check-clang-extra'], attempting to kill
...
PASS: Clang Tools :: clang-doc/DR-141990.cpp (58 of 1516)
PASS: Clang Tools :: clang-doc/typedef-alias.cpp (59 of 1516)
PASS: Clang Tools :: clang-doc/DR-131697.cpp (60 of 1516)
PASS: Clang Tools :: clang-apply-replacements/identical.cpp (61 of 1516)
PASS: Clang Tools :: clang-apply-replacements/crlf.cpp (62 of 1516)
PASS: Clang Tools :: clang-doc/json/concept.cpp (63 of 1516)
PASS: Clang Tools :: clang-doc/mustache-separate-namespace.cpp (64 of 1516)
PASS: Clang Tools :: clang-doc/conversion_function.cpp (65 of 1516)
PASS: Clang Tools :: clang-change-namespace/argument-parsing-error-no-abort.cpp (66 of 1516)
PASS: Clang Tools :: clang-reorder-fields/AggregatePartialInitialization.c (67 of 1516)
command timed out: 1200 seconds without output running [b'ninja', b'check-clang-extra'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1254.882929

```

</details>

https://github.com/llvm/llvm-project/pull/204748