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).
A copy of the older Concurrency sources was just added to BackDeployConcurrency,
replacing the `add_subdirectory(../Concurrency)` trick it was using previously.
These need to be adjusted to work with the threading library. Fortunately, the
necessary adjustments are the same as those required for the existing
Concurrency library, give or take.
rdar://90776105
