[Preamble] Stop circular inclusion of main file when building preamble
If a header file was processed for the second time, we could end up with a
wrong conditional stack and skipped ranges:
In the particular example, if the header guard is evaluated the second time and
it is decided to skip the conditional block, the corresponding "#endif" is
never seen since the preamble does not include it and we end up in the
Tok.is(tok::eof) case with a wrong conditional stack.
Detect the circular inclusion, emit a diagnostic and stop processing the