When using the parser to do static analysis and editor integrations, it's
still useful to be able to get the incomplete AST resulting from a parse
error.
Callers that intend to use the returned value to take real actions (as
opposed to just analysis) must check diags.HasError() to determine if
the returned file can be considered valid.
Even if errors were encountered during parsing, it is helpful to still
return a partial AST if possible since that allows for the source code
analysis features to still (partially) work in the face of errors.
