[flang][driver] Disallow non-existent input files in the frontend driver

This patch adds a check that verifies that the input file used when
calling the frontend driver (i.e. `flang-new -fc1`) actually exists.
This was not required for the compiler driver, `flang-new`, as that's
already handled in libclangDriver.

Once all input/output file management is moved to the driver, we should
also check that for input from `stdin` the corresponding file descriptor
was successfully acquired.

This patch also makes sure that the default action in the frontend is
`ParseSyntaxOnly`. This is consistent with Clang. Before this change
`flang-new -fc1` would do nothing, which makes testing changes like the
one introduced here a bit tricky.

Reviewed By: SouraVX

Differential Revision: https://reviews.llvm.org/D95127