Code cleanup: Add isMain() accessor to FunctionDeclaration.

This returns true if the function name is `main`. We have special
treatment for functions named `main` in various places throughout the
compiler, so this accessor comes in handy fairly often.

Change-Id: I9da17648800f0385875e1e7d66aacb5f3dea3817
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/388443
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed