Extend commit 96a953b1ed (Add options to specify linker tool, 2023-09-27,
v3.29.0-rc1~577^2) with the `CMAKE_Swift_USING_LINKER_*` variables
needed to support `CMAKE_LINKER_TYPE` for Swift.
When building on Windows, pass `-debug` to the linker to emit the PDB
(or `-debug:dwarf` when building with lld and using DWARF). We would
previously not do this which meant that we never emitted the debug
information making debugging more difficult.
Fixes: #24423
This moves the definition of the import library flags for Windows into a
platform specific file (much like the Darwin `-sdk` flag). This is in
preparation for re-use of the flag to make the flags handling for
executables more precise.
Evan Wilde