[flang] Recognize END FILE as ENDFILE in free form source

[klausler]

Summary

The ENDFILE statement may be spelled as two words.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/lib/Parser/io-parsers.cpp

 // R1225 endfile-stmt ->
 //         ENDFILE file-unit-number | ENDFILE ( position-spec-list )
 TYPE_CONTEXT_PARSER("ENDFILE statement"_en_US,
-    construct<EndfileStmt>("ENDFILE" >> positionOrFlushSpecList))
+    construct<EndfileStmt>("END FILE" >> positionOrFlushSpecList))
 
 // R1226 rewind-stmt -> REWIND file-unit-number | REWIND ( position-spec-list )
 TYPE_CONTEXT_PARSER("REWIND statement"_en_US,

flang/test/Semantics/call10.f90

     close(1) ! C1597
     !ERROR: External I/O is not allowed in a pure subprogram
     backspace(1) ! C1597
+    !Also checks parsing of variant END FILE spelling
     !ERROR: External I/O is not allowed in a pure subprogram
-    endfile(1) ! C1597
+    end file(1) ! C1597
     !ERROR: External I/O is not allowed in a pure subprogram
     rewind(1) ! C1597
     !ERROR: External I/O is not allowed in a pure subprogram