[flang][runtime] ensure character compares to blank are unsigned

[jeanPerier]

Summary

CompareToBlankPadding was doing signed compare on architecture where
char is signed. This caused 'abc'//char(128) > 'abc' to evaluate
to false at runtime instead of true.

[PeteSteinfeld]

Other than my comment about repeated code, all build, tests, and looks good.

flang/runtime/character.cpp

25–30

You could avoid some repeated code here:

UNSIGNED_CHAR ux = *reinterpret_cast<const UNSIGNED_CHAR *>(x);
if (ux < blank) {
  return -1;
}
if (ux > blank) {

flang/runtime/character.cpp

 #include "flang/Runtime/descriptor.h"
 #include <algorithm>
 #include <cstring>
 
 namespace Fortran::runtime {
 
 template <typename CHAR>
 inline int CompareToBlankPadding(const CHAR *x, std::size_t chars) {
+  using UNSIGNED_CHAR = std::make_unsigned_t<CHAR>;
+  const auto blank{static_cast<UNSIGNED_CHAR>(' ')};
   for (; chars-- > 0; ++x) {
-    if (*x < ' ') {
+    const UNSIGNED_CHAR ux{*reinterpret_cast<const UNSIGNED_CHAR *>(x)};
+    if (ux < blank) {
       return -1;
     }
-    if (*x > ' ') {
+    if (ux > blank) {
       return 1;
     }
   }
   return 0;
 }
 
 template <typename CHAR>
 int CharacterScalarCompare(

flang/unittests/Runtime/CharacterTest.cpp

using ComparisonTestCasesTy = std::tuple<ComparisonParametersTy<char>,
     ComparisonParametersTy<char16_t>, ComparisonParametersTy<char32_t>>;
 
 static ComparisonTestCasesTy comparisonTestCases{
     {
         std::make_tuple("abc", "abc", 3, 3, 0),
         std::make_tuple("abc", "def", 3, 3, -1),
         std::make_tuple("ab ", "abc", 3, 2, 0),
         std::make_tuple("abc", "abc", 2, 3, -1),
+        std::make_tuple("ab\xff", "ab ", 3, 2, 1),
+        std::make_tuple("ab ", "ab\xff", 2, 3, -1),
     },
     {
         std::make_tuple(u"abc", u"abc", 3, 3, 0),
         std::make_tuple(u"abc", u"def", 3, 3, -1),
         std::make_tuple(u"ab ", u"abc", 3, 2, 0),
         std::make_tuple(u"abc", u"abc", 2, 3, -1),
     },
     {