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
Xcode and other build systems currently link the compatibility libraries into every executable
or dynamic library, which can cause a chain of objc_getClass compatibility hacks to pile up if
a program loads a lot of dynamic libraries. In the static constructor that installs the
objc_getClass compatibility shim, check whether it's running on behalf of the main executable
before installing the shim.
