[flang] Extension: allow tabs in output format strings

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.

Differential Revision: https://reviews.llvm.org/D112350

flang/docs/Extensions.md

@@ -122,6 +122,7 @@ end
   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

flang/runtime/format-implementation.h

@@ -359,6 +359,10 @@ int FormatControl<CONTEXT>::CueUpNextDataEdit(Context &context, bool stop) {
       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));