[flang] Missing diagnostic on `SIGN=` specifier being used on unformatted I/O

[DanielCChen]

Consider the following code:

open(5, file='sign03.tmp', form="unformatted", sign='suppress')
end

The standard only allows the SIGN= specifier on formatted I/O as:

The SIGN= specifier is permitted only for a connection for formatted input/output.

This case should be diagnosed.

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Daniel Chen (DanielCChen)

Consider the following code: ``` open(5, file='sign03.tmp', form="unformatted", sign='suppress') end ```

The standard only allows the SIGN= specifier on formatted I/O as:

The SIGN= specifier is permitted only for a connection for formatted input/output.

This case should be diagnosed.

[klausler]

Use IOSTAT= or ERR= and you will observe an error in this case. In the absence of error recovery, I don't crash the program for an error like this that can safely be ignored.