[flang] Fix crash from -DMACRO= with empty replacement

[klausler]

Summary

Such macros were exposing some edge cases in the preprocessor
regarding empty tokens.

[PeteSteinfeld]

Builds, tests, and looks good.

flang/include/flang/Common/interval.h

void ExtendToCover(const Interval &that) {
     }
   }
 
   std::size_t MemberOffset(const A &x) const {
     CHECK(Contains(x));
     return x - start_;
   }
   A OffsetMember(std::size_t n) const {
-    CHECK(n < size_);
+    CHECK(n <= size_);
     return start_ + n;
   }
 
   constexpr A Last() const { return start_ + (size_ - 1); }
   constexpr A NextAfter() const { return start_ + size_; }
   constexpr Interval Prefix(std::size_t n) const {
     return {start_, std::min(size_, n)};
   }

flang/lib/Parser/provenance.cpp

 
 void OffsetToProvenanceMappings::Put(const OffsetToProvenanceMappings &that) {
   for (const auto &map : that.provenanceMap_) {
     Put(map.range);
   }
 }
 
 ProvenanceRange OffsetToProvenanceMappings::Map(std::size_t at) const {
-  //  CHECK(!provenanceMap_.empty());
+  if (provenanceMap_.empty()) {
+    CHECK(at == 0);
+    return {};
+  }
   std::size_t low{0}, count{provenanceMap_.size()};
   while (count > 1) {
     std::size_t mid{low + (count >> 1)};
     if (provenanceMap_[mid].start > at) {
       count = mid - low;
     } else {
       count -= mid - low;
       low = mid;