[Flang] Add a warning message for the `CONVERT=` specifier appear on an OPEN statement for a formatted file.

[DanielCChen]

The CONVERT= specifier is not supported on an OPEN statement for a formatted file.
Flang is currently silence about it.
Add a warning message for the following case.

open(unit=3, form='formatted', convert='LITTLE_ENDIAN')

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Daniel Chen (DanielCChen)

The `CONVERT=` specifier is not supported on an OPEN statement for a formatted file. Flang is currently silence about it. Add a warning message for the following case. ``` open(unit=3, form='formatted', convert='LITTLE_ENDIAN') ```

[tarunprabhu]

As of bba40ab, flang will produce the following warning, but only if -pedantic is used:

$ flang -pedantic -c -o /dev/null 94236.f90
94236.f90:2:32: portability: nonstandard usage: CONVERT=
  open(unit=3, form='formatted', convert='LITTLE_ENDIAN')
                                 ^^^^^^^^^^^^^^^^^^^^^^^
94236.f90:2:1: in the context: OPEN statement
  open(unit=3, form='formatted', convert='LITTLE_ENDIAN')
  ^
94236.f90:1:1: in the context: main program
  program hello
  ^

@DanielCChen, is this sufficient? If so, can this issue be closed?

[DanielCChen]

Yes. Thank you!
This works for me.