[llvm] Add macOS premerge check workflow (PR #207435)

[Louis Dionne via llvm-commits]

================
@@ -132,19 +132,26 @@
 
 EXCLUDE_MAC = {
     "bolt",
-    "compiler-rt",
+    "CIR",  # Depends on mlir, which is excluded below.
     "cross-project-tests",
     "flang",
+    "flang-rt",
     "libc",
-    "lldb",
+    "mlir",
     "openmp",
     "polly",
     "libcxx",
     "libcxxabi",
-    "libunwind",
----------------
ldionne wrote:

> Dropping libunwind here does not change whether or not we run the libunwind tests through the monolithic premerge on libunwind changes given we don't set that up in `DEPENDENTS_TO_TEST`.

Ack, I didn't know that. My preference would still be not to change that in this patch and then actually submit a PR where we can discuss the situation more exhaustively. I don't disagree with the end goal you're after, I actually think it's better than the status quo. It's just that I'd like to consider the situation in its entirety and I think that's better done separately.

I care about this, but not enough to block this patch, so if you feel very strongly that we should make this change in this PR, I won't fight it.

https://github.com/llvm/llvm-project/pull/207435