[flang] Search for #include "file" in right directory (take 2)

[flang] Search for #include "file" in right directory (take 2)

Make the #include "file" preprocessing directive begin its
search in the same directory as the file containing the directive,
as other preprocessors and our Fortran INCLUDE statement do.

Avoid current working directory for all source files except the original.

Resolve tests.

Differential Revision: https://reviews.llvm.org/D95481