Fix generation of 'isa' and 'discriminator' keywords.

Summary:
There should be a space before each of these two keywords to avoid
generating invalid assembly files.

NOTE: I could not find an obvious maintainers in CODE_OWNERS.TXT, but
      this seems related to debug info.

Reviewers: echristo

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2791