The naming convention is different on Windows than on Unix-like
environments. In order to follow the convention we need to substitute
the prefix and the suffix. Take the opportunity to rename the
`target-dylib-extension` to the CMake-like variable
`target-shared-library-suffix` and introduce
`target-shared-library-prefix`. This helps linking the test suite
binaries on Windows.
A resilient change should not *remove* previously-public symbols.
Note that currently, the test_superclass_properties test does not
meet this critierion, because we always emit keypath property
descriptors, even when the property is an override. Fixing this
is a larger change that I'll address in a follow-on PR, so for now
I'm just going to disable the symbol check for this one test only.
Part of <rdar://problem/40432647>.
With a few modifications:
- Changing computed get-only properties into 'let's breaks resilience in other ways we need to fix independent of key path resilience
- Fix the tests not to ask for impossible lib-before--client-after compatibility for changes that add API
