[CodeGen] Replace '@' characters in block descriptors' symbol names with

'\1'.

'@' 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.

See the discussion here: https://reviews.llvm.org/D50783.

Differential Revision: https://reviews.llvm.org/D54539

llvm-svn: 350157