libexpr: Convert Value::mkStringMove to a constructor

The pseudo-constructor `Value::mkStringMove` would previously be called
on a default-constructed (under-initialized) `Value` to create a
properly initialized `Value` that represents a string, reusing an
existing GC allocated Str. This change turns that method into a proper
constructor

Change-Id: I8d7a97e2afce231a04ecf56fdd6d22d06a6a6964

lix/libexpr/eval-expr.cc

@@ -541,7 +541,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         }
         v = {NewValueAs::path, CanonPath(canonPath(str()))};
     } else {
-        v.mkStringMove(gcStr(), context);
+        v = {NewValueAs::string, gcStr(), context};
     }
 }
 

lix/libexpr/value.cc

@@ -74,7 +74,7 @@ void Value::mkString(std::string_view s, const NixStringContext & context)
     copyContextToValue(*untag<String *>(), context);
 }
 
-void Value::mkStringMove(Str * s, const NixStringContext & context)
+Value::Value(string_t, Str * s, const NixStringContext & context)
 {
     auto block = gcAllocType<String>();
     *block = {.content = s, .context = nullptr};

lix/libexpr/value.hh

@@ -447,6 +447,12 @@ public:
         raw = tag(tString, block);
     }
 
+    /// Constructs a nix language value of type "string", with the value of the
+    /// C-string pointed to by @ref s, and context from @ref context.
+    ///
+    /// The C-string is not copied but the data from context is
+    Value(string_t, Str * s, const NixStringContext & context);
+
     Value(string_t, const String * str) : raw(tag(tString, str)) {}
 
     /// Constructx a nix language value of type "string", with a copy of the
@@ -784,8 +790,6 @@ public:
 
     void mkString(std::string_view s, const NixStringContext & context);
 
-    void mkStringMove(Str * s, const NixStringContext & context);
-
     inline void mkNull()
     {
         *this = {NewValueAs::null};