[flang] Avoid crash case in provenance mapping

[klausler]

Summary

When a contiguous range of a cooked character stream is being
mapped to a range of source provenance, the code was assuming
that the "end()" position of the input range -- being the character
immediately after the range -- would also follow the range's
source provenance. This isn't always the case.

Modify the code to work with the true last character of the
input range (at end()-1) and to also cope with cases when that
last position truly maps to an earlier provenance, which can happen
when the prescanner has inserted a space into the cooked character
stream.

[mleair]

LGTM

flang/lib/Parser/provenance.cpp

 }
 
 std::optional<ProvenanceRange> CookedSource::GetProvenanceRange(
     CharBlock cookedRange) const {
   if (!AsCharBlock().Contains(cookedRange)) {
     return std::nullopt;
   }
   ProvenanceRange first{provenanceMap_.Map(cookedRange.begin() - &data_[0])};
-  if (cookedRange.size() <= first.size()) {
+  if (cookedRange.size() <= first.size()) { // always true when empty
     return first.Prefix(cookedRange.size());
   }
-  ProvenanceRange last{provenanceMap_.Map(cookedRange.end() - &data_[0])};
-  return {ProvenanceRange{first.start(), last.start() - first.start()}};
+  ProvenanceRange last{provenanceMap_.Map(cookedRange.end() - 1 - &data_[0])};
+  if (first.start() <= last.start()) {
+    return {ProvenanceRange{first.start(), last.start() - first.start() + 1}};
+  } else {
+    return std::nullopt;
+  }
 }
 
 std::optional<CharBlock> CookedSource::GetCharBlock(
     ProvenanceRange range) const {
   CHECK(!invertedMap_.empty() &&
       "CompileProvenanceRangeToOffsetMappings not called");
   if (auto to{invertedMap_.Map(range)}) {
     return CharBlock{data_.c_str() + *to, range.size()};