llvm: rebuild

* llvm: add mlir
  - [x] Have you followed the [guidelines for contributing](https://github.com/Homebrew/homebrew-core/blob/HEAD/CONTRIBUTING.md)?
  - [x] Have you checked that there aren't other open [pull requests](https://github.com/Homebrew/homebrew-core/pulls) for the same formula update/change?
  - [ ] Have you built your formula locally with `brew install --build-from-source <formula>`, where `<formula>` is the name of the formula you're submitting?
  - [ ] Is your test running fine `brew test <formula>`, where `<formula>` is the name of the formula you're submitting?
  - [ ] Does your build pass `brew audit --strict <formula>` (after doing `brew install <formula>`)?

  There's been some interest in the flang compiler for llvm:

  Homebrew/discussions#143
  #63081

  It seems possible to build flang separately from llvm, but it appears to
  require having built mlir. See the flang [README](https://github.com/llvm/llvm-project/blob/master/flang/README.md).

  My laptop is too sadly underpowered to build llvm. I will try to devise
  a test if this is built successfully. I think I should also be able to
  add a separate formula for flang if this is successful.
* llvm: use cmake to invoke make indirectly
  Building llvm with mlir succeeded on my local machine with a similar
  procedure (except using ninja), so hopefully this works.
* llvm: add test for mlir

Closes #65223.

Signed-off-by: FX Coudert <[email protected]>

Formula/llvm.rb

@@ -79,6 +79,7 @@ def install
       lldb
       openmp
       polly
+      mlir
     ]
     runtimes = %w[
       compiler-rt
@@ -139,9 +140,9 @@ def install
     llvmpath = buildpath/"llvm"
     mkdir llvmpath/"build" do
       system "cmake", "-G", "Unix Makefiles", "..", *(std_cmake_args + args)
-      system "make"
-      system "make", "install"
-      system "make", "install-xcode-toolchain" if MacOS::Xcode.installed?
+      system "cmake", "--build", "."
+      system "cmake", "--build", ".", "--target", "install"
+      system "cmake", "--build", ".", "--target", "install-xcode-toolchain" if MacOS::Xcode.installed?
     end
 
     # Install LLVM Python bindings
@@ -209,6 +210,15 @@ def caveats
       }
     EOS
 
+    # Testing mlir
+    (testpath/"test.mlir").write <<~EOS
+      func @bad_branch() {
+        br ^missing  // expected-error {{reference to an undefined block}}
+      }
+    EOS
+
+    system "#{bin}/mlir-opt", "--verify-diagnostics", "test.mlir"
+
     # Testing default toolchain and SDK location.
     system "#{bin}/clang++", "-v",
            "-std=c++11", "test.cpp", "-o", "test++"