[flang][runtime] Accept "." as REAL input

[klausler]

Summary

".", possibly followed by an exponent, is a valid REAL input value (meaning zero).

flang/runtime/edit-input.cpp

if (next && *next == '(') { // NaN(...)
       }
     }
     exponent = 0;
   } else if (first == decimal || (first >= '0' && first <= '9') ||
       (bzMode && (first == ' ' || first == '\t')) || first == 'E' ||
       first == 'D' || first == 'Q') {
     Put('.'); // input field is normalized to a fraction
     auto start{got};
-    bool anyDigit{false};
     for (; next; next = io.NextInField(remaining, edit)) {
       char32_t ch{*next};
       if (ch == ' ' || ch == '\t') {
         if (bzMode) {
           ch = '0'; // BZ mode - treat blank as if it were zero
         } else {
           continue;
         }
       }
       if (ch == '0' && got == start && !decimalPoint) {
-        anyDigit = true;
         // omit leading zeroes before the decimal
       } else if (ch >= '0' && ch <= '9') {
-        anyDigit = true;
         Put(ch);
       } else if (ch == decimal && !decimalPoint) {
         // the decimal point is *not* copied to the buffer
         decimalPoint = got - start; // # of digits before the decimal point
       } else {
         break;
       }
     }
     if (got == start) {
-      if (anyDigit) {
-        Put('0'); // emit at least one digit
-      } else {
-        return 0; // no digits, invalid input
-      }
+      // Nothing but zeroes and maybe a decimal point.  F'2018 requires
+      // at least one digit, but F'77 did not, and a bare "." shows up in
+      // the FCVS suite.
     }
     if (next &&
         (*next == 'e' || *next == 'E' || *next == 'd' || *next == 'D' ||
             *next == 'q' || *next == 'Q')) {
       // Optional exponent letter.  Blanks are allowed between the
       // optional exponent letter and the exponent value.
       io.SkipSpaces(remaining);
       next = io.NextInField(remaining, edit);