When the generator parameter 'annotate_code' is provided, generate a .meta
file containing a GeneratedCodeInfo message describing the generated code's
relation to the source .proto file.
Annotations are added with (*protogen.GeneratedFile).Annotate, which takes the
name of a Go identifier (e.g., "SomeMessage" or "SomeMessage.GetField") and an
associated source location. The generator examines the generated AST to
determine source offsets for the symbols.
Change the []int32 "Path" in protogen types to a "Location", which also captures
the source file name.
Change-Id: Icd2340875831f40a1f91d495e3bd7ea381475c77
Reviewed-on: https://go-review.googlesource.com/c/139759
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
