error: 'error' diagnostics seen but not expected: File /share/dim/src/llvm/trunk/tools/clang/test/CXX/drs/dr6xx.cpp Line 259: static_assert failed "__STDC_MB_MIGHT_NEQ_WC__ but all basic source characters have same representation" 1 error generated. See the discussion here: http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-January/080416.html
Fixed in r225751. Defining this macro is the wrong thing for a compiler to do, according to the relevant standards, but it's conforming, so let's just put up with it and move on.