crystal: depend on llvm@6 instead of llvm

llvm 7.0.0 introduced some breaking changes that are not supported by crystal 0.26.1

Closes #32441.

Signed-off-by: commitay <commitay@users.noreply.github.com>

Formula/crystal.rb

@@ -1,6 +1,7 @@
 class Crystal < Formula
   desc "Fast and statically typed, compiled language with Ruby-like syntax"
   homepage "https://crystal-lang.org/"
+  revision 1
 
   stable do
     url "https://github.com/crystal-lang/crystal/archive/0.26.1.tar.gz"
@@ -33,7 +34,7 @@ class Crystal < Formula
   depends_on "gmp" # std uses it but it's not linked
   depends_on "libevent"
   depends_on "libyaml"
-  depends_on "llvm"
+  depends_on "llvm@6"
   depends_on "pcre"
 
   resource "boot" do