Widen AST bitfields too small to represent all enumerators

All of these were found by a new warning that I am prototyping,
-Wbitfield-enum-conversion.

Stmt::ExprBits::ObjectKind - This was not wide enough to represent
OK_ObjSubscript, so this was a real, true positive bug.

ObjCDeclSpec::objCDeclQualifier - On Windows, setting DQ_CSNullability
would cause the bitfield to become negative because enum types are
always implicitly 'int' there. This would probably never be noticed
because this is a flag-style enum, so we only ever test one bit at a
time. Switching to 'unsigned' also makes this type pack smaller on
Windows.

FunctionDecl::SClass - Technically, we only need two bits for all valid
function storage classes. Functions can never have automatic or register
storage class. This seems a bit too clever, and we have a bit to spare,
so widening the bitfield seems like the best way to pacify the warning.
You could classify this as a false positive, but widening the bitfield
defends us from invalid ASTs.

llvm-svn: 297680