[clang] [clang] Fix darwin-related tests' REQUIRES annotation (PR #130138)

[Lucas Duarte Prates via cfe-commits]

pratlucas wrote:

For context, we're observing this issue as part of our CI builds for the [arm/arm-toolchain](https://github.com/arm/arm-toolchain) project.

Looking into it in more detail, I believe I didn't express myself correctly when I wrote "When no darwin
targets are included in the build". The actual scenario we have in our toolchain is that we explicitly set the value of `LLVM_DEFAULT_TARGET_TRIPLE` in our CMake files.
With this, the starting target triple used by clang won't be related to darwin, even when running on a darwin host platform. (Note that this is intentional, as our toolchains are only designed to target embedded and linux environments)

> This doesn't happen for tests with target=<arch>-apple<platform&version> right? It's not clear to me why the tests that explicitly pass an alternative way to set the target triple (e.g. with env vars or with mtargetos) fail. It would be good to diagnose that first.

The issue doesn't happen if the `--target` option is used. This happens because the `-arch` and `-mtargetos` options don't control the entire target triple, but only modify specific parts of it (arch and OS). They are also only considered when the original target is a darwin one, taking no effect otherwise:

- The processing of the `-arch` option is guarded by the triple's `isOSBinFormatMachO()` method in the driver.
- The `-mtargetos=` option is only ever evaluated with the Darwin Toolchain implementation, as part of `Darwin::AddDeploymentTarget`. This is only reachable if the Darwin Toolchain is used by clang, which in turn requires a darwin target triple.

Given the above, I think it's reasonable to expect that these tests will only pass when the default target triple is a darwin one, matching the updated `REQUIRES` annotations.

> Do you have a bot or build to point to for test failures?

Unfortunately our CI runs are not public yet, but here's the error output for reference:

```
<src_path>/clang/test/Driver/apple-arm64-arch.c:6:11: error: CHECK: expected string not found in input
// CHECK: "-triple" "arm64-apple-macosx{{[0-9.]+}}"
          ^
<stdin>:1:1: note: scanning from here
clang version 21.0.0git
^
<stdin>:8:89: note: possible intended match here
 "<src_path>/build/llvm/bin/clang-20" "-cc1" "-triple" "aarch64-unknown-linux-gnu" "-emit-obj" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "apple-arm64-arch.c" "-mrelocation-model" "pic" "-pic-level" "2" "-pic-is-pie" "-mframe-pointer=non-leaf" "-fmath-errno" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-funwind-tables=2" "-enable-tlsdesc" "-target-cpu" "generic" "-target-feature" "+v8a" "-target-feature" "+fp-armv8" "-target-feature" "+neon" "-target-abi" "aapcs" "-debugger-tuning=gdb" "-fdebug-compilation-dir=<src_path>/build/llvm/tools/clang/test/Driver" "-target-linker-version" "1115.7.3" "-fcoverage-compilation-dir=<src_path>/build/llvm/tools/clang/test/Driver" "-resource-dir" "<src_path>/build/llvm/lib/clang/21" "-internal-isystem" "<src_path>/build/llvm/lib/clang/21/include" "-internal-isystem" "/usr/local/include" "-internal-externc-isystem" "/include" "-internal-externc-isystem" "/usr/include" "-ferror-limit" "19" "-fno-signed-char" "-fgnuc-version=4.2.1" "-fskip-odr-check-in-gmf" "-target-feature" "-fmv" "-faddrsig" "-D__GCC_HAVE_DWARF2_CFI_ASM=1" "-o" "apple-arm64-arch.o" "-x" "c" "<src_path>/clang/test/Driver/apple-arm64-arch.c"
                                                                                        ^

Input file: <stdin>
Check file: <src_path>/clang/test/Driver/apple-arm64-arch.c

-dump-input=help explains the following input dump.

Input was:
<<<<<<
           1: clang version 21.0.0git 
check:6'0     X~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
           2: Target: aarch64-unknown-linux-gnu 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           3: Thread model: posix 
check:6'0     ~~~~~~~~~~~~~~~~~~~~
           4: InstalledDir: <src_path>/build/llvm/bin 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           5: Arm Toolchain ID: E0000-pre (a6d8b454) 
check:6'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           6: clang: error: unsupported option '-arch' for target 'aarch64-linux-gnu' 
```

https://github.com/llvm/llvm-project/pull/130138