Fix handling of preincrement on bit-fields. This gives a bit-field in C++, but

we were failing to find that bit-field when performing integer promotions. This
brings us closer to following the standard, and closer to GCC.

In C, this change is technically a regression: we get bit-field promotions
completely wrong in C, promoting cases that are categorically not bit-field
designators. This change makes us do so slightly more consistently, though.

llvm-svn: 218428