CI: Use LDC-LLVM v15.0.7 for prebuilt packages

[kinke]

No description provided.

[kinke]

Changes for that new v15.0.7 on our end:

• Now includes a macOS-arm64 build with enabled assertions, which is now used for the Cirrus macOS arm64 (native) CI jobs.
• Travis failed badly to finish the Linux-aarch64 build in under 50 minutes (their hard timeout). So I've ported the Travis jobs to Cirrus CI, using native Linux-aarch64 containers with 8 CPU cores (LLVM build itself done in ~30 minutes; max job timeout: 2 hours).
• I've excluded the Mach-O hack to preserve the .debug_line DWARF section.

[kinke]

We seem to be hitting a new assertion with -cov in general, which should account for most failures (Attribute::isEnumAttrKind(Kind) && "Not an enum attribute").

And lit-test codegen/dcompute_cl_images.d still hits the same assertion as in #4010 (comment). FYI @thewilsonator

[kinke]

The only remaining regression/blocker is codegen/dcompute_cl_images.d.

[kinke]

And looks like the macOS-arm64 crt_destructor issues for dmd-testsuite (Cirrus CI) are gone with LLVM 15.

[kinke]

Oh and 1 more TODO: checking for new CLI options inherited from LLVM 15, and hiding those.

[kinke]

@thewilsonator: I went with marking tests/codegen/dcompute_cl_images.d as XFAIL with LLVM 15+ for now, as suggested in #4010 (comment).

[kinke]

tests/sanitizers/fuzz_asan.d seems flaky again, sigh... (LLVM 15 only?)