libexpr: Clear PosTable contents in EvalState::resetFileCache

Otherwise PosTable grows indefinitely for each reload. Since
the total input size is limited to 4GB (uint32_t for byte offset PosIdx)
it can get exhausted pretty. This ensures that we don't waste memory
on reloads as well.

src/libexpr/eval.cc

@@ -1113,6 +1113,7 @@ void EvalState::resetFileCache()
     importResolutionCache->clear();
     fileEvalCache->clear();
     inputCache->clear();
+    positions.clear();
 }
 
 void EvalState::eval(Expr * e, Value & v)

src/libutil/include/nix/util/pos-table.hh

@@ -111,6 +111,16 @@ public:
             return o->origin;
         return std::monostate{};
     }
+
+    /**
+     * Remove all origins from the table.
+     */
+    void clear()
+    {
+        auto lines = linesCache.lock();
+        lines->clear();
+        origins.clear();
+    }
 };
 
 } // namespace nix