update chapel-main homebrew formula for 2.3 (#25998)

This updates the homebrew formula we use to test main against.
Homebrew's default LLVM package was recently updated to 19 and we don't
yet support it, so this pins our dep to 18.

This is a trivial change to our testing formula only, not reviewed.

util/packaging/homebrew/chapel-main.rb

@@ -14,7 +14,7 @@ class Chapel < Formula
   depends_on "gmp"
   depends_on "hwloc"
   depends_on "jemalloc"
-  depends_on "llvm"
+  depends_on "llvm@18"
   depends_on "pkg-config"
   depends_on "[email protected]"
 
@@ -70,10 +70,10 @@ def install
       system "make", "cleanall"
 
       rm_r("third-party/llvm/llvm-src/")
-      rm_r("third-party/gasnet/gasnet-src")
-      rm_r("third-party/libfabric/libfabric-src")
+      rm_r("third-party/gasnet/gasnet-src/")
+      rm_r("third-party/libfabric/libfabric-src/")
       rm_r("third-party/fltk/fltk-1.3.8-source.tar.gz")
-      rm_r("third-party/libunwind/libunwind-src")
+      rm_r("third-party/libunwind/libunwind-src/")
       rm_r("third-party/gmp/gmp-src/")
       rm_r("third-party/qthread/qthread-src/")
     end