[clang][Interp][NFC] Return a valid SourceInfo for Function PCs

We already assert that the given PC is in range and that the
function has a body, so the SrcMap should generally never be empty.
However, when generating destructors, we create quite a few instructions
for which we have no source information, which may cause the previous
assertion to fail. Return the end of the source map in this case.

Author: tbaederr

clang/lib/AST/Interp/Function.cpp

@@ -40,7 +40,8 @@ SourceInfo Function::getSource(CodePtr PC) const {
   unsigned Offset = PC - getCodeBegin();
   using Elem = std::pair<unsigned, SourceInfo>;
   auto It = llvm::lower_bound(SrcMap, Elem{Offset, {}}, llvm::less_first());
-  assert(It != SrcMap.end());
+  if (It == SrcMap.end())
+    return SrcMap.back().second;
   return It->second;
 }