[Clang] Rename -split-dwarf-file to -split-dwarf-output

Summary:
This is the first in a series of changes trying to align clang -cc1
flags for Split DWARF with those of llc. The unfortunate side effect of
having -split-dwarf-output for single file Split DWARF will disappear
again in a subsequent change.

The change is the result of a discussion in D59673.

Reviewers: dblaikie, echristo

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D63130

llvm-svn: 363494