Rename split_style::DOT

This renames split_style::DOT, to avoid name clashes when building gdb
with an old version of Bison (2.3, the version available on macOS).

In particular the error looks like:

./split-name.h:34:3: error: expected identifier
  DOT,
  ^
m2-exp.c:163:13: note: expanded from macro 'DOT'

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30286