The runtime that shipped with Swift 5.1 and earlier had a bug that interfered with backward
deployment of binaries that dynamically check for protocol conformances on conditionally-available
tests. This was fixed in the top-of-tree Swift runtime by https://github.com/apple/swift/pull/29887;
however, that doesn't do much good for running binaries on older OSes that don't have that fix.
In order for binaries built with a newer Swift compiler to run successfully on older OSes,
introduce a compatibility hook that replaces the conformance cache implementation in the original
OS runtime with a version based on the current implementation that has the fix for the protocol
conformance bug. Fixes rdar://problem/59460603
When building on macOS without the standard library but building the
extra toolchain content, we would fail to configure due to the missing
include of the `AddSwiftStdlib`.
