[flang] Rework read/write permission management for runtime file opening

Anonymous Fortran unit files (e.g., "./fort.7") need to be created
O_RDWR so that they can be written, rewound, and read.  Other
files opened with no ACTION= specifier need to set read/write
permissions based on the file, if it exists.

Reviewed By: sscalpone

Differential Revision: https://reviews.llvm.org/D84063