Fix OS X linking warnings due to a roaming llvm symbol.
Fix OS X linking warnings due to a roaming llvm symbol.

This fixes the OS X warnings like this one:

 ld: warning: direct access in function 'XXX' from file
 'libLLVMScalarOpts.a(NewGVN.cpp.o)' to global weak
 symbol 'llvm::ReverseIterate<bool>::value' from file
 'interpreter/llvm/src/lib/libclingUtils.a(AST.cpp.o)'
 means the weak symbol cannot be overridden
 at runtime. This was likely caused by different translation
 units being compiled with different visibility settings.

I assume it's a compiler bug and it maybe fixes itself in LLVM 6.0
as this is quite recently introduced code, so let's go with the
most conservative fix and just disable this validation layer in LLVM
(that we don't use from what I can see).

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