Swift: Restore support for enabling with INTERFACE libraries

The check added in commit b06f4c8a (Swift: disallow WIN32_EXECUTABLE
properties, 2019-05-31, v3.15.0-rc1~9^2) makes sense only for
executables because the `WIN32_EXECUTABLE` property is defined only for
them.  Running the check on other target types, particularly those that
do not link such as INTERFACE libraries, violates internal assumptions.
In particular, `GetLinkerLanguage` should not be called on such targets.

Fixes: #19528
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