Make Unix.h:MakeErrMsg separate the prefix and errno string, so we get:

  clang: error: unable to make temporary file: /etc/cc: can't make
  unique filename: Permission denied

instead of 

  clang: error: unable to make temporary file: /etc/cc: can't make
  unique filenamePermission denied

for example.

Also, audited the uses of MakeErrMsg to make the prefix strings
consistent (not end with newline/punctuation/space/": ").

llvm-svn: 69626