Make sure NVPTX doesn't emit symbol names that aren't valid in PTX.

NVPTX, like the other backends, relies on generic symbol name sanitizing done by
MCSymbol. However, the ptxas assembler is more stringent and disallows some
additional characters in symbol names.

See PR19099 for more details.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203483 91177308-0d34-0410-b5e6-96231b3b80d8