[flang][runtime] Ignore leading spaces even in BZ mode

[klausler]

Summary

When editing numeric input, always skip leading spaces, even if
BZ mode (or BLANK='ZERO') is in effect; otherwise, a sign character
preceded by blanks will not be recognized.

flang/runtime/edit-input.cpp

 static inline char32_t GetDecimalPoint(const DataEdit &edit) {
   return edit.modes.editingFlags & decimalComma ? char32_t{','} : char32_t{'.'};
 }
 
 // Prepares input from a field, and consumes the sign, if any.
 // Returns true if there's a '-' sign.
 static bool ScanNumericPrefix(IoStatementState &io, const DataEdit &edit,
     std::optional<char32_t> &next, std::optional<int> &remaining) {
-  bool bzMode{(edit.modes.editingFlags & blankZero) != 0};
-  next = io.PrepareInput(edit, remaining, !bzMode);
+  next = io.PrepareInput(edit, remaining);
   bool negative{false};
   if (next) {
     negative = *next == '-';
     if (negative || *next == '+') {
-      if (!bzMode) {
-        io.SkipSpaces(remaining);
-      }
       next = io.NextInField(remaining, edit);
     }
   }
   return negative;
 }
 
 bool EditIntegerInput(
     IoStatementState &io, const DataEdit &edit, void *n, int kind) {

flang/runtime/io-stmt.h

public:
   std::optional<char32_t> GetCurrentChar(std::size_t &byteCount);
 
   bool EmitRepeated(char, std::size_t);
   bool EmitField(const char *, std::size_t length, std::size_t width);
 
   // For fixed-width fields, initialize the number of remaining characters.
   // Skip over leading blanks, then return the first non-blank character (if
   // any).
-  std::optional<char32_t> PrepareInput(const DataEdit &edit,
-      std::optional<int> &remaining, bool skipSpaces = true) {
+  std::optional<char32_t> PrepareInput(
+      const DataEdit &edit, std::optional<int> &remaining) {
     remaining.reset();
     if (edit.descriptor == DataEdit::ListDirected) {
       std::size_t byteCount{0};
       GetNextNonBlank(byteCount);
     } else {
       if (edit.width.value_or(0) > 0) {
         remaining = *edit.width;
       }
-      if (skipSpaces) {
-        SkipSpaces(remaining);
-      }
-    }
     return NextInField(remaining, edit);
   }
 
   std::optional<char32_t> SkipSpaces(std::optional<int> &remaining) {
     while (!remaining || *remaining > 0) {
       std::size_t byteCount{0};
       if (auto ch{GetCurrentChar(byteCount)}) {
         if (*ch != ' ' && *ch != '\t') {