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

[Eugene Epshteyn via llvm-commits]

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@@ -1128,6 +1128,44 @@ 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.
+
+These arguments must all be integers.
+The value returned from the function form of `FTELL` is `INTEGER(8)`.
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eugeneepshteyn wrote:

I'm ok with it being `INTEGER(8)`, but please note that both GNU and nvfortran documentation have it as default INTEGER.

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