protoc: trailing comment on last line of file (with no subsequent newline) ignored

[jhump]

Apparently I introduced a bug in #10660. 🤦

If the last line in the file has a trailing comment and no trailing newline, that comment is effectively ignored (it gets interpreted as a detached comment before the EOF instead of a trailing comment on the preceding token).

Here's a simple example:

syntax = "proto3";
package foo.bar; // trailing comment on package.

Importantly, if the file above has no trailing newline (i.e. the last byte in the file is the period of the trailing comment), then that comment does not make it into source code info. One would expect it to be a trailing comment for the package statement.