[AST] Update/correct the static_asserts for the bit-fields in Type

The current static_assert only checks that ObjCObjectTypeBitfields
fits into an unsigned. However it turns out that FunctionTypeBitfields
do not currently fits into an unsigned. Therefore the anonymous
union containing the bit-fields always use 8 bytes instead of 4.

This patch removes the lone misguided static_assert and systematically
checks the size of each bit-field.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D50630

llvm-svn: 339582