CheckSourceCompiles: For Swift executable, name source 'main.swift'

Xcode uses its own heuristics to determine whether or not to accept
top-level code in a source file while Ninja uses the swift driver
heuristics.

With the Swift driver, if the module contains a single file, that file
will be parsed as a top-level code context. With Xcode, the single file
will only be parsed as top-level code if the name of that file is
'main.swift'.

To ensure more consistent behavior between the two generators, if we're
building Swift and the try-compile target type is executable or
undefined, we name the file `main.swift` to ensure that both will handle
the single file as top-level code.

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