llvm: add config file, update caveats

Adding a system config file will allow us to stop baking in the
`DEFAULT_SYSROOT` and allow users on Xcode-only installations to use the
bottles. It will also make it easier to update this value when users
upgrade the major version of their macOS. Before this change, this
required a full reinstall. After, users will simply have to
`brew postinstall llvm`.

The config file change requires an upstream patch. We'll probably have
to wait a little bit for it to merge, and then wait another few days to
make sure it doesn't get reverted.

Following discussion at #192505, let's also update the caveats to notify
users of potential changes regarding `LLVM_ENABLE_EH` in LLVM 20.

Formula/l/llvm.rb

@@ -22,9 +22,6 @@ class Llvm < Formula
     sha256 cellar: :any_skip_relocation, x86_64_linux:  "4066cc4bba868660b63f4130073b19d551b82d2cbeb9fe6d96e5a68d864ee305"
   end
 
-  # Clang cannot find system headers if Xcode CLT is not installed
-  pour_bottle? only_if: :clt_installed
-
   keg_only :provided_by_macos
 
   # https://llvm.org/docs/GettingStarted.html#requirement
@@ -54,6 +51,18 @@ def python3
     "python3.12"
   end
 
+  def clang_config_file_dir
+    etc/"clang"
+  end
+
+  # Backport relative `CLANG_CONFIG_FILE_SYSTEM_DIR` patch.
+  # Remove in LLVM 20.
+  # https://github.com/llvm/llvm-project/pull/110962
+  patch do
+    url "https://github.com/llvm/llvm-project/commit/e31545f666eb4ca32030956a38dbc4078a64068c.patch?full_index=1"
+    sha256 "53c2af0027e06ec0268ddda205c28aecf065ea08374dba0d8be573aa0d1b5aa7"
+  end
+
   def install
     # The clang bindings need a little help finding our libclang.
     inreplace "clang/bindings/python/clang/cindex.py",
@@ -127,6 +136,7 @@ def install
       -DCLANG_PYTHON_BINDINGS_VERSIONS=#{python_versions.join(";")}
       -DLLVM_CREATE_XCODE_TOOLCHAIN=OFF
       -DCLANG_FORCE_MATCHING_LIBCLANG_SOVERSION=OFF
+      -DCLANG_CONFIG_FILE_SYSTEM_DIR=#{clang_config_file_dir.relative_path_from(bin)}
     ]
 
     if tap.present?
@@ -157,7 +167,6 @@ def install
       args << "-DLIBCXX_INSTALL_LIBRARY_DIR=#{libcxx_install_libdir}"
       args << "-DLIBUNWIND_INSTALL_LIBRARY_DIR=#{libunwind_install_libdir}"
       args << "-DLIBCXXABI_INSTALL_LIBRARY_DIR=#{libcxx_install_libdir}"
-      args << "-DDEFAULT_SYSROOT=#{macos_sdk}" if macos_sdk
       runtimes_cmake_args << "-DCMAKE_INSTALL_RPATH=#{libcxx_rpaths.join("|")}"
 
       # Disable builds for OSes not supported by the CLT SDK.
@@ -446,6 +455,18 @@ def install
     end
   end
 
+  def post_install
+    return unless OS.mac?
+    return unless (macos_sdk = MacOS.sdk_path_if_needed)
+
+    clang_config_file_dir.mkpath
+    %w[clang.cfg clang++.cfg].each do |config_file|
+      (clang_config_file_dir/config_file).atomic_write <<~CONFIG
+        --sysroot=#{macos_sdk}
+      CONFIG
+    end
+  end
+
   def caveats
     s = <<~EOS
       `lld` is now provided in a separate formula:
@@ -462,6 +483,12 @@ def caveats
           LDFLAGS="-L#{opt_lib}/c++ -L#{opt_lib}/unwind -lunwind"
 
         NOTE: You probably want to use the libunwind and libc++ provided by macOS unless you know what you're doing.
+
+        When upgrading to a new major version of macOS (e.g. 15 to 16), please run:
+          brew postinstall llvm
+
+        We plan to build LLVM 20 with `LLVM_ENABLE_EH=OFF`. Please see:
+          <link to issue or discussion>
       EOS
     end