Revert "[flang] Search for #include "file" in right directory"

This reverts commit d987b61b.

As pointed out in https://reviews.llvm.org/D95388, the reverted commit
causes build failures in the following Flang buildbots:
  * http://lab.llvm.org:8011/#/builders/32/builds/2642
  * http://lab.llvm.org:8011/#/builders/33/builds/2131
  * http://lab.llvm.org:8011/#/builders/135/builds/1473
  * http://lab.llvm.org:8011/#/builders/66/builds/1559
  * http://lab.llvm.org:8011/#/builders/134/builds/1409
  * http://lab.llvm.org:8011/#/builders/132/builds/1817
I'm guessing that the patch was only tested with
`FLANG_BUILD_NEW_DRIVER=Off` (i.e. the default). The builders listed
above set `FLANG_BUILD_NEW_DRIVER` to `On`.

Although fixing the build is relatively easy, the reverted patch
modifies the behaviour of the frontend, which breaks driver tests. In
particular, in https://reviews.llvm.org/D93453 support for `-I` was
added that depends on the current behaviour. The reverted patch
changes that behaviour. Either the tests have to be updated or the
change fine-tuned.