Add the missing `ObjCShims.h` header to the installed header set.
Without this the module is incomplete. This was detected on Windows when
trying to build the SwiftShims module.
When building the standard library statically on Windows, we should not
use any __declspec(dllimport) or __declspec(dllexport) on the runtime
functions or data. Furthermore, we should internalise all of the
standard library functions so that they are not re-exported if used to
build a shared library. The code generation changes required for this
are easier to identify once a static SDK is available for Windows.
This patch hooks up the swiftCore library build and gets it installing.
This means that we have library evolution hooked up and vector types.
Building it dynamically found that we had duplicate symbols, so fixed
the swiftDemanglingCR library.
Needed to hook up the availability macros.
The flag configuration is for macOS, so we'll need to go through and
figure out what it should look like for the other platforms too.
