Ninja: Emit swiftmodule from executable with exports

This patch adds support for tracking the swiftmodules for executables
exporting symbols.

This fixes a bug in the earlier implementation around emitting the
swiftmodule. Previously, the code would use
`CMAKE_EXE_EXPORTS_Swift_FLAG` to inject the `-emit-module`, and module
path information into the `CMAKE_Swift_LINK_EXECUTABLE` rule. Because
Swift skips the build step and only runs during the link phase, these
flags were injected in `cmNinjaNormalTargetGenerator::ComputeLinkCmd`
instead of `cmLocalGenerator::GetTargetFlags` where it is done normally.

Unfortunately, injecting in `ComputeLinkCmd` didn't do anything because
we have a `linkCmd` so `ComputeLinkCmd` exits early, before the
EXE_EXPORT flags get added to the link command.

Instead of playing with that flag, CMake checks
`CMAKE_Swift_LINK_EXECUTABLE_WITH_EXPORTS` and uses that as the link
rule if it exists and falls back on `CMAKE_Swift_LINK_EXECUTABLE`. I've
defined that variable in terms of `CMAKE_Swift_LINK_EXECUTABLE` with the
necessary additional flags for emitting the swift module instead. This
has the same end effect as the desired behavior, but simplifies things a
bit.

Since we're generating the swiftmodule for executables with exports,
I've also updated the dependency graph to include the swiftmodule as an
output in the build dependency graph if the executable has exports.

Tests updated:
 - RunCMake/NoWorkToDo:
   Ensure that the build graph does not result in unnecessary rebuilds
   with exporting executables.

 - SwiftOnly:
   Ensure that we can consume functions defined in the executable by a
   library getting linked into said executable.

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