Make module naming consistent with the integrated driver when the output filename has 2+ extensions

[owenv]

This caused a surprising number of tests to fail which wrote something like -o a.out.optimized. The C++ driver uses llvm::sys::path::stem to remove extensions, which would result in a.out, which is an invalid module name, so the fallback of main would be used. swift-driver used to remove both extensions, and would name the module a. This behavior is clearly better, but I think it's too late to change considering how many tests broke...

[DougGregor]

Sigh, compatibility. This is a good fix to match the behavior of the current driver, and it's not important enough to break compatibility over. Any real build tool will always pass down -module-name explicitly anyway.

[DougGregor]

@swift-ci please test