cmd/compile/internal/pgo: add hint to missing start_line error

Profiles only began adding Function.start_line in 1.20. If it is
missing, add a hint to the error message that they may need to profile a
build of the application built with a newer version of the toolchain.

Technically profiles are not required to come from Go itself (e.g., they
could be converted from perf), but in practice they most likely are.

Fixes #57674.

Change-Id: I87eca126d3fed0cff94bbb8dd748bd4652f88b12
Reviewed-on: https://go-review.googlesource.com/c/go/+/461195
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: Michael Pratt <[email protected]>

Author: prattmic

src/cmd/compile/internal/pgo/irgraph.go

@@ -217,7 +217,7 @@ func (p *Profile) processprofileGraph(g *Graph) bool {
 		// TODO(prattic): If Function.start_line is missing we could
 		// fall back to using absolute line numbers, which is better
 		// than nothing.
-		log.Fatal("PGO profile missing Function.start_line data")
+		log.Fatal("PGO profile missing Function.start_line data (Go version of profiled application too old? Go 1.20+ automatically adds this to profiles)")
 	}
 
 	return true