Add the inreg attribute to sreg when present.
On Windows/AArch64, a different register is used between when an
arugment is both inreg and sret (X0 or X1) and when it is just sret
(X8) as the following comment indicates:
https://github.com/llvm/llvm-project/blob/46fe36a4295f05d5d3731762e31fc4e6e99863e9/llvm/lib/Target/AArch64/AArch64CallingConvention.td#L42
```
// In AAPCS, an SRet is passed in X8, not X0 like a normal pointer parameter.
// However, on windows, in some circumstances, the SRet is passed in X0 or X1
// instead. The presence of the inreg attribute indicates that SRet is
// passed in the alternative register (X0 or X1), not X8:
// - X0 for non-instance methods.
// - X1 for instance methods.
// The "sret" attribute identifies indirect returns.
// The "inreg" attribute identifies non-aggregate types.
// The position of the "sret" attribute identifies instance/non-instance
// methods.
// "sret" on argument 0 means non-instance methods.
// "sret" on argument 1 means instance methods.
CCIfInReg<CCIfType<[i64],
CCIfSRet<CCIfType<[i64], CCAssignToReg<[X0, X1]>>>>>,
CCIfSRet<CCIfType<[i64], CCAssignToReg<[X8]>>>,
```
So missing/dropping inreg can cause a codegen bug.
This is a partial fix for #74866
Cherrypick https://github.com/swiftlang/swift/pull/76159
