Re-add clang-format-15 formula from #4

Formula/clang-format@15.rb

@@ -0,0 +1,82 @@
+class ClangFormatAT15 < Formula
+  desc "Formatting tools for C, C++, Obj-C, Java, JavaScript, TypeScript"
+  homepage "https://clang.llvm.org/docs/ClangFormat.html"
+  # The LLVM Project is under the Apache License v2.0 with LLVM Exceptions
+  license "Apache-2.0"
+  version_scheme 1
+  head "https://github.com/llvm/llvm-project.git", branch: "main"
+
+  stable do
+    url "https://github.com/llvm/llvm-project/releases/download/llvmorg-15.0.7/llvm-15.0.7.src.tar.xz"
+    sha256 "4ad8b2cc8003c86d0078d15d987d84e3a739f24aae9033865c027abae93ee7a4"
+
+    resource "clang" do
+      url "https://github.com/llvm/llvm-project/releases/download/llvmorg-15.0.7/clang-15.0.7.src.tar.xz"
+      sha256 "a6b673ef15377fb46062d164e8ddc4d05c348ff8968f015f7f4af03f51000067"
+    end
+
+    resource "cmake" do
+      url "https://github.com/llvm/llvm-project/releases/download/llvmorg-15.0.7/cmake-15.0.7.src.tar.xz"
+      sha256 "8986f29b634fdaa9862eedda78513969fe9788301c9f2d938f4c10a3e7a3e7ea"
+    end
+  end
+
+  livecheck do
+    url :stable
+    regex(%r{href=.*?/tag/llvmorg[._-]v?(\d+(?:\.\d+)+)}i)
+    strategy :github_latest
+  end
+
+  depends_on "cmake" => [:build, :test]
+
+  uses_from_macos "python" => [:build, :test], since: :catalina
+  uses_from_macos "libxml2"
+  uses_from_macos "ncurses"
+  uses_from_macos "zlib"
+
+  on_linux do
+    keg_only "it conflicts with llvm"
+  end
+
+  def install
+    llvmpath = if build.head?
+      ln_s buildpath/"clang", buildpath/"llvm/tools/clang"
+
+      buildpath/"llvm"
+    else
+      (buildpath/"src").install buildpath.children
+      (buildpath/"src/tools/clang").install resource("clang")
+      (buildpath/"cmake").install resource("cmake")
+
+      buildpath/"src"
+    end
+
+    system "cmake", "-S", llvmpath, "-B", "build",
+                    "-DLLVM_EXTERNAL_PROJECTS=clang",
+                    "-DLLVM_INCLUDE_BENCHMARKS=OFF",
+                    *std_cmake_args
+    system "cmake", "--build", "build", "--target", "clang-format"
+
+    bin.install "build/bin/clang-format" => "clang-format-15"
+    bin.install llvmpath/"tools/clang/tools/clang-format/git-clang-format" => "git-clang-format-15"
+    # (share/"clang").install llvmpath.glob("tools/clang/tools/clang-format/clang-format*")
+  end
+
+  test do
+    system "git", "init"
+    system "git", "commit", "--allow-empty", "-m", "initial commit", "--quiet"
+
+    # NB: below C code is messily formatted on purpose.
+    (testpath/"test.c").write <<~EOS
+      int         main(char *args) { \n   \t printf("hello"); }
+    EOS
+    system "git", "add", "test.c"
+
+    assert_equal "int main(char *args) { printf(\"hello\"); }\n",
+        shell_output("#{bin}/clang-format-15 -style=Google test.c")
+
+    # Fails with 'error: cannot find executable "clang-format"'.
+    # ENV.prepend_path "PATH", bin
+    # assert_match "test.c", shell_output("git clang-format-15", 1)
+  end
+end