cmd/vet: false positive for build flag when it appears in a string of source code

[srinathh]

I ran into a likely false positive from go vet when writing a program that auto-generates code.

This program generates a warning +build comment must appear before package clause and be followed by a blank line

This equivalent program without the // autogenerated by... comment in the first line doesn't

In either case, i don't think go vet should be vetting contents of the string,

[robpike]

Unfortunate.

The algorithm it uses is very simple, but is fooled by this corner case. Not worth the trouble to fix.