Don't use an invalid srcInfo to construct json source info.

[ChrisDodd]

• Also fix off-by-one error tracking stdin line number

I think there are more off-by-one errors in the linenumber tracking code, combined with various +1/-1 adjustments in random places to correct for them. Much of this comes from #line directives containing the line number of the next line, not the line number of the #line directive itself.

[ChrisDodd]

The code for getting the source fragment of assignments also seems broken to me.

[jafingerhut]

I am aware that the code for getting the source fragment of assignments often produces weird results, at least for assignments synthesized by the compiler.

It does produce good results for most assignments that appear in the original source code. I'm happy to open a separate issue to remember that those should be improved in some way.

[ChrisDodd]

In general, you should be checking the SourceInfo objects with isValid() before trying to map them back to source code -- when isValid() is false, there's no source information (usually because the node was created by the compiler where it had no source info to put in it, but sometimes just because it failed to provide that argument to the constructor).

[ChrisDodd]

In addition, you can "concatenate" SourceInfo objects with +. If both are valid the resulting SourceInfo will be from the start of the first one to the end of the second. If only one is valid, that one will be returned unchanged.

[jafingerhut]

I haven't dug into the code around the change to the call to mapLine() to understand that change, but I have tried out this patch on the small public program that demonstrates the line == 2^32-1 issue, and it comes out better on that one, as well as on a private program I cannot share that had the same problem.

[jafingerhut]

@ChrisDodd Thanks for the tip on "+" being defined for SourceInfo objects. I have a patch to the assignment code that uses that technique instead, which I think improves it. I will wait until this one is committed before opening a PR on that, if that is OK.