Fix the check for regparm in FunctionType::ExtInfo

`getHasRegParm()` was working under the assumption that the RegParm
bits are the last field, which is no longer true, after adding the
`NoCfCheck` and `CmseNSCall` fields.

This causes a spurious "regparm 0" attribute to appear when a function
type is declared with either `__attribute__((nocf_check))` or
`__attribute__((cmse_nonsecure_call))`.

Differential Revision: https://reviews.llvm.org/D77270