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).
This is for the 'freestanding' build to stop assuming the platform has argc/argv.
- Introduce a new sub-library, libswiftCommandLineSupport.a
- Move stubs/CommandLine.cpp into this library
- Conditionally embed it into libswiftCore
- Conditionally embed it into libswiftPrivateLibcExtras if not in libswiftCore to support testing
- Add SWIFT_STDLIB_HAS_COMMANDLINE CMake (and build-script) flag
