The _Builtin_float symbols were moved twice, most recently from the OS Darwin library, but previously they were in the embedded @rpath Darwin library. @_originallyDefinedIn doesn't support multiple install names, but it can be replaced with -module-abi-name, and then multiple $ld$previous$ symbols can be used.
Update the Platform and Concurrency magic symbols to use $ld$previous$ everywhere.
rdar://130107191
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).
With these magic symbols, programs that link against the _Concurrency
module with a deployment target prior to iOS 15 / macOS 12 / watchOS 8
will reference `libswift_Concurrency.dylib` via rpath.
Fixes rdar://81187835.
