Don't report "main" as missing a prototype in freestanding mode

Summary:
A user may want to use freestanding mode with the standard "main" entry
point. It's not useful to warn about a missing prototype as it's not
typical to have a prototype for "main".

Reviewers: efriedma, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: aaron.ballman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70588