Install swiftmodules with full module triple

[etcwilde]

Use full module triple in the installed swiftmodules.

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

Motivation:

Clean up how the CMake build installs swiftmodules, using the full module triple instead of just the architecture. This change uses the compiler to compute the expected module triple instead of attempting to compute it in CMake script. This is more maintainable and robust to adding new platforms and architectures.

Modifications:

Update the CMake build to query the compiler for the full module triple instead of attempting to compute it in CMake script.

[glbrntt]

Thanks @etcwilde