Restore size of TemplateParameterList after D44352

After adding a field of one bit, the bitfield members would take
30+1+1+1 = 33 bits, causing the size of TemplateParameterList to
increase from 16 to 24 bytes on 64-bit systems.

With 29 bits for NumParams we can encode up to half a billion template
parameters, which is almost certainly still enough for anybody.