swift234

SILGen: Emit "main" as a SIL function.

Eliminate the intermediate top_level_code function. Now that SIL is expressive enough to express a "main" function, there's no reason for it, and this eliminates a bunch of mystery code in IRGen to thunk from main to top_level_code by reaching for hardcoded symbol names. Demystify the special code for setting up C_ARGC and C_ARGV by having SILGen look for a transparent "_didEnterMain" hook in the stdlib and emit a call to it.

Swift SVN r22525