swift234

[Frontend] Parse the main file last to make sure implicit visibility works.

Otherwise, we'll try to type-check bits of the main source file before we've
even looked at any of the supporting files.

This affects implicit multi-file mode, where "main.swift" is assumed to be
the main source file and all others are treated as library files. (See r9890.)

<rdar://problem/15526743>

Swift SVN r10795