build: pin LLVM 22.1.8 toolchain

Author: helantianshen

mcpp.toml

@@ -8,5 +8,8 @@ license     = "Apache-2.0"
 authors     = ["mcpplibs"]
 repo        = "https://github.com/mcpplibs/cmp"
 
+[toolchain]
+default = "llvm@22.1.8"
+
 [dev-dependencies.compat]
 gtest = "1.15.2"