clang-format: Don't insert spaces in front of :: for Java 8 Method References.

The existing code kept the space if it was there for identifiers, and it didn't
handle `this`. After this patch, for Java `this` is handled in addition to
identifiers, and existing space is always stripped between identifier and `::`.

Also accept `::` in addition to `.` in front of `<` in `foo::<T>bar` generic
calls.

Differential Revision: https://reviews.llvm.org/D52842

llvm-svn: 343872