[AST] Move the enum in ObjCMethodDeclBitfields

Move the enum { ObjCMethodFamilyBitWidth = 4 } to the top of
the class. For some dark reason having the enum between the
bitfields breaks the packing with gcc version 7.3-win32 20180312.

Reported by: Abramo Bagnara (by email)

llvm-svn: 339017