[flang] Extend ProvenanceRange::Suffix() to handle crash case

[klausler]

Summary

Suffix() can be called from AllSources::IntersectionWithSourceFiles()
when a contiguous range of source provenance overlaps a macro expansion.
It skips over the macro expansion and recurses on the remainder of
the range, which might end with a bit that does overlap with a
source file. However, in the case where the original range is
entirely within the expanded macro, Suffix() crashes when called
with a skip offset greater than the size of the range.

Rather than add logic around this and other calls to Suffix() to
avoid passing an out-of-range skip, it's better to accommodate it
in Suffix() and return an empty result.

flang/include/flang/Common/interval.h

public:
   }
 
   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)};
   }
   Interval Suffix(std::size_t n) const {
-    CHECK(n <= size_);
+    n = std::min(n, size_);
     return {start_ + n, size_ - n};
   }
 
   constexpr Interval Intersection(const Interval &that) const {
     if (that.NextAfter() <= start_) {
       return {};
     } else if (that.start_ <= start_) {
       auto skip{start_ - that.start_};