cmake: also check Homebrew install paths when looking for LLVM

- On Intel Macs, the path is /usr/local/opt/llvm@12
- On Silicon Macs, the path is /opt/homebrew/opt/llvm@12

This makes specifying CMAKE_PREFIX_PATH optional for Homebrew LLVM.

cmake/Findllvm.cmake

@@ -14,6 +14,8 @@ find_path(LLVM_INCLUDE_DIRS NAMES llvm/IR/IRBuilder.h
     /usr/lib/llvm-12.0/include
     /usr/local/llvm12/include
     /usr/local/llvm120/include
+    /usr/local/opt/llvm@12/include
+    /opt/homebrew/opt/llvm@12/include
     /mingw64/include
 )
 
@@ -31,6 +33,8 @@ if(ZIG_PREFER_CLANG_CPP_DYLIB)
       /usr/lib/llvm-12/lib
       /usr/local/llvm12/lib
       /usr/local/llvm120/lib
+      /usr/local/opt/llvm@12/lib
+      /opt/homebrew/opt/llvm@12/lib
   )
 
   find_program(LLVM_CONFIG_EXE
@@ -185,6 +189,8 @@ else()
       /usr/lib/llvm-12.0/lib
       /usr/local/llvm120/lib
       /usr/local/llvm12/lib
+      /usr/local/opt/llvm@12/lib
+      /opt/homebrew/opt/llvm@12/lib
       /mingw64/lib
       /c/msys64/mingw64/lib
       c:\\msys64\\mingw64\\lib)