[Clang] Remove unused -split-dwarf and obsolete -enable-split-dwarf

Summary:
The changes in D59673 made the choice redundant, since we can achieve
single-file split DWARF just by not setting an output file name.
Like llc we can also derive whether to enable Split DWARF from whether
-split-dwarf-file is set, so we don't need the flag at all anymore.

The test CodeGen/split-debug-filename.c distinguished between having set
or not set -enable-split-dwarf with -split-dwarf-file, but we can
probably just always emit the metadata into the IR.

The flag -split-dwarf wasn't used at all anymore.

Reviewers: dblaikie, echristo

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D63167

llvm-svn: 364479