configure: Allow use of LLVM 20

(cherry picked from commit ca03226)

Author: bgamari

configure.ac

@@ -536,7 +536,7 @@ AC_SUBST(InstallNameToolCmd)
 # versions of LLVM simultaneously, but that stopped working around
 # 3.5/3.6 release of LLVM.
 LlvmMinVersion=13  # inclusive
-LlvmMaxVersion=20  # not inclusive
+LlvmMaxVersion=21  # not inclusive
 AC_SUBST([LlvmMinVersion])
 AC_SUBST([LlvmMaxVersion])