[flang] [llvm] [flang] Implement FSEEK and FTELL (PR #133003)

[Eugene Epshteyn via llvm-commits]

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@@ -1128,6 +1128,41 @@ program chdir_func
 end program chdir_func
 ```
 
+### Non-Standard Intrinsics: FSEEK and FTELL
+
+#### Description
+`FSEEK(UNIT, OFFSET, WHENCE)` Sets position in file opened as `UNIT`, returns status.
+
+`CALL FSEEK(UNIT, OFFSET, WHENCE[, STATUS])` Sets position, returns any error in `STATUS` if present.
+
+`FTELL(UNIT)` Returns current absolute byte offset.
+
+`CALL FTELL(UNIT, OFFSET)` Set `OFFSET` to current byte offset in file.
+
+These intrinsic procedures are available as both functions and subroutines,
+but both forms cannot be used in the same scope.
+
+|            |                                                 |
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eugeneepshteyn wrote:

Not clear what the types of the args are. This is especially important for `FTELL()`, which returns a default integer and is not suitable for files larger than 2GB in size.

https://github.com/llvm/llvm-project/pull/133003