Darwin defines memcmp with optional pointers. Update SwiftShims to
define it to the same type to avoid deserialization failures where we
get one over the other and the types don't match anymore.
rdar://140596571
macOS 15.0, iOS 18.0, et al has started unconditionally declaring `lgamma_r` in <math.h>, add a __has_include in SwiftShims to detect that and not redeclare it when building against those SDK versions.
rdar://115192929
Mostly this just means adding `Musl` as a module dependency of various
things and making sure that we build things for `swift_static` even
if `SWIFT_BUILD_STATIC_STDLIB` isn't enabled.
There's also a slight difference in the declaration of `memcmp()`;
musl's declaration is more like the one we have on Darwin.
rdar://123508245
This replaces a number of `#include`-s like this:
```
#include "../../../stdlib/public/SwiftShims/Visibility.h"
```
with this:
```
#include "swift/shims/Visibility.h"
```
This is needed to allow SwiftCompilerSources to use C++ headers which include SwiftShims headers. Currently trying to do that results in errors:
```
swift/swift/include/swift/Demangling/../../../stdlib/public/SwiftShims/module.modulemap:1:8: error: redefinition of module 'SwiftShims'
module SwiftShims {
^
Builds.noindex/swift/swift/bootstrapping0/lib/swift/shims/module.modulemap:1:8: note: previously defined here
module SwiftShims {
^
```
This happens because the headers in both the source dir and the build dir refer to SwiftShims headers by relative path, and both the source root and the build root contain SwiftShims headers (which are equivalent, but since they are located in different dirs, Clang treats them as different modules).
