[Shims] Move `Visibility.h` under Swift include path
I am trying to remove more of the SILBridging logic and switch SwiftCompilerSources/SIL to C++ interop.
When importing the `SIL/SILInstruction.h` C++ header into Swift, we end up with duplicate module definition for SwiftShims:
```
/Volumes/Projects/swift/swift/include/swift/Demangling/../../../stdlib/public/SwiftShims/module.modulemap:1:8: error: redefinition of module 'SwiftShims'
module SwiftShims {
^
/Users/egorzh/Builds.noindex/swift/swift/bootstrapping0/lib/swift/shims/module.modulemap:1:8: note: previously defined here
module SwiftShims {
^
```
One of the modules comes from the Swift project sources, and another one from the just-built shims. This happens because `SILInstruction.h` transitively includes `SwiftShims/Visibility.h`, adding another include path that points to SwiftShims from the Swift sources.
To prevent that, let's move the header from SwiftShims to `include/swift/Basic` and introduce a trivial `SwiftShims/Visibility.h` header for compatibility.