databases/mongodb50: stick to <= llvm18 on FreeBSD 15

build is broken with llvm19

fixes:
[00:35:47] In file included from src/third_party/mozjs-60/platform/aarch64/freebsd/build/Unified_cpp_js_src19.cpp:2:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/jit/MacroAssembler.cpp:7:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/jit/MacroAssembler-inl.h:10:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/jit/MacroAssembler.h:14:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/vm/JSCompartment.h:22:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/gc/Zone.h:18:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/vm/Runtime.h:27:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/frontend/NameCollections.h:13:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/vm/Stack.h:23:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/jit/JSJitFrameIter.h:15:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/vm/JSFunction.h:17:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/vm/JSScript.h:31:
[00:35:48] In file included from src/third_party/mozjs-60/extract/js/src/vm/SharedImmutableStringsCache.h:21:
[00:35:48] src/third_party/mozjs-60/extract/js/src/threading/ExclusiveData.h:124:33: error: reference to non-static member function must be called
[00:35:48] 124 | : lock_(mozilla::Move(rhs.lock))
[00:35:48] | ~~~~^~~~