The driver actually adds a default -mlinker-version, based on HOST_LINK_VERSION
cmake variable. The tests should be explicit about which version they're using to
trigger the right behavior.
In Xcode 11, ld added a new flag called -platform_version that can be used instead of the old -<platform>_version_min flags.
The new flag allows Clang to pass the SDK version from the driver to the linker.
This patch adopts the new -platform_version flag in Clang, and starts using it by default,
unless a linker version < 520 is passed to the driver.
Differential Revision: https://reviews.llvm.org/D71579
Alex Lorenz