clang-7.0 currently on the buildbot is broken on 10.7

[kencu (Ken)]

$ /opt/local/bin/clang++-mp-7.0
Stack dump:
0.	Program arguments: /opt/local/libexec/llvm-7.0/bin/clang++ 
0  libLLVM.dylib     0x000000010b2ead49 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  libLLVM.dylib     0x000000010b2ea691 llvm::sys::RunSignalHandlers() + 51
2  libLLVM.dylib     0x000000010b2eb19c SignalHandler(int) + 119
3  libsystem_c.dylib 0x00007fff8e3cbcfa _sigtramp + 26
4  libsystem_c.dylib 0x0000003900000012 _sigtramp + 1908622130
5  clang++           0x0000000109c5fda0 llvm::iterator_range<llvm::opt::arg_iterator<llvm::opt::Arg* const*, sizeof...(clang::driver::options::ID, clang::driver::options::ID)> > llvm::opt::ArgList::filtered<clang::driver::options::ID, clang::driver::options::ID>(clang::driver::options::ID, clang::driver::options::ID) const + 68
6  clang++           0x0000000109c5d728 llvm::opt::Arg* llvm::opt::ArgList::getLastArg<clang::driver::options::ID, clang::driver::options::ID>(clang::driver::options::ID, clang::driver::options::ID) const + 36
7  clang++           0x0000000109c43dfc clang::ParseDiagnosticArgs(clang::DiagnosticOptions&, llvm::opt::ArgList&, clang::DiagnosticsEngine*, bool, bool) + 202
8  clang++           0x00000001098ce158 main + 5261
9  clang++           0x00000001098ccb48 start + 52
10 clang++           0x0000000000000001 start + 4134745325
Segmentation fault: 11

most likely we need to rebuild llvm-7.0 with a different clang compiler than the one that is currently building it.