Use clang50, clang++50 to build lang/ponyc

Building the bitcode with clang70 and linking with llvm50 fails,
so let's use the clang from the LLVM version that will be used by
ponyc.

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p490918_s343288/logs/errors/ponyc-0.25.0_1.log

PR: 234290
Submitted by: greg@unrelenting.technology (maintainer)