This fixes a bug introduced in r340041 (see the discussion in https://reviews.llvm.org/D50783).
'@' can't be used in block descriptors' symbol names since it is reserved on ELF platforms as a separator between symbol names and symbol versions.
I don't know of any forbidden symbols on Windows PE/COFF symbols, but if there are, I can make changes similar to the ones I made in this patch.