[CIR][ABI] Add _BitInt flag to IntType (#188113)
Add an optional `bitint` parameter to `cir::IntType` so CIR can
distinguish `_BitInt(N)` from builtin integer types like `__int128`.
Both lower to the same LLVM integer type, but they have different ABI
rules (alignment, passing convention) on x86_64.
The flag is set during CIRGen for `Type::BitInt`, printed/parsed as
`!cir.int<s, 128, bitint>`, and excluded from `isFundamental()`.
Existing 2-arg `IntType::get()` calls continue to work via a default
parameter.
