[Clang] Limit FunctionTypeExtraBitfields::NumExceptionType to 16 bits.

In https://reviews.llvm.org/D127762#4102578 @erichkeane suggested to
limit size of this field to 16bits, such that the field that encodes the
SME attributes for a function fall within the alignment of the struct for
32bit platforms.

Standard implimits defines the minimum handlers per try block to 256,
which suggests that 16bits should be more than sufficient for most
programs. Erich also pointed out that exception specs are being
deprecated and are rarely used, so hopefully this change is safe to make.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D152140