This patch changes the behaviour for flang to only create and link to a
`main` entry point when the Fortran code has a program statement in it.
This means that flang-new can be used to link even when the program is
a mixed C/Fortran code with `main` present in C and no entry point
present in Fortran.
This also removes the `-fno-fortran-main` flag as this no longer has any
functionality.
8d5386669e