[flang] Extension: allow tabs in output format strings

[klausler]

Summary

A CHARACTER variable used as an output format may contain
unquoted tab characters, which are treated as if they had
been quoted. This is an extension supported by all other
Fortran compilers to which I have access.

flang/docs/Extensions.md

* Character literals as elements of an array constructor without an explicit
   type specifier need not have the same length; the longest literal determines
   the length parameter of the implicit type, not the first.
 * Outside a character literal, a comment after a continuation marker (&)
   need not begin with a comment marker (!).
 * Classic C-style /*comments*/ are skipped, so multi-language header
   files are easier to write and use.
 * $ and \ edit descriptors are supported in FORMAT to suppress newline
   output on user prompts.
+* Tabs in format strings (not `FORMAT` statements) are allowed on output.
 * REAL and DOUBLE PRECISION variable and bounds in DO loops
 * Integer literals without explicit kind specifiers that are out of range
   for the default kind of INTEGER are assumed to have the least larger kind
   that can hold them, if one exists.
 * BOZ literals can be used as INTEGER values in contexts where the type is
   unambiguous: the right hand sides of assigments and initializations
   of INTEGER entities, and as actual arguments to a few intrinsic functions
   (ACHAR, BTEST, CHAR).  BOZ literals are interpreted as default INTEGER

flang/runtime/format-implementation.h

if (ch == '(') {
         }
         HandleControl(context, static_cast<char>(ch), static_cast<char>(next),
             repeat ? *repeat : 1);
       }
     } else if (ch == '/') {
       context.AdvanceRecord(repeat && *repeat > 0 ? *repeat : 1);
     } else if (ch == '$' || ch == '\\') {
       context.mutableModes().nonAdvancing = true;
+    } else if (ch == '\t' || ch == '\v') {
+      // Tabs (extension)
+      // TODO: any other raw characters?
+      context.Emit(format_ + offset_ - 1, 1);
     } else {
       context.SignalError(IostatErrorInFormat,
           "Invalid character '%c' in FORMAT", static_cast<char>(ch));
       return 0;
     }
   }
 }