stdlib: Fix warning about superfluous 'const' on return type.
Resolves the following warnings:
```
stdlib/public/SwiftShims/swift/shims/CoreFoundationShims.h:86:1: warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]
86 | const _swift_shims_NSUInteger
| ^~~~~
stdlib/public/stubs/FoundationHelpers.mm:110:1: warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]
110 | const _swift_shims_NSUInteger
| ^~~~~
```
