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

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

Differential Revision: https://reviews.llvm.org/D101207

flang/include/flang/Common/interval.h

@@ -93,7 +93,7 @@ public:
     return x - start_;
   }
   A OffsetMember(std::size_t n) const {
-    CHECK(n < size_);
+    CHECK(n <= size_);
     return start_ + n;
   }
 

flang/lib/Parser/provenance.cpp

@@ -91,7 +91,10 @@ void OffsetToProvenanceMappings::Put(const OffsetToProvenanceMappings &that) {
 }
 
 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)};