Fix false positive for "variable is mutated but never accessed"

When value or variable is assigned a result of an operation such as
"and" or "or", it is externally accessible and mutations of it
may have an effect even if it is not directly accessed. Same
if value node tag is Paren (e.g. `local t = (f())`).

Ref #72.

spec/check_spec.lua

@@ -345,6 +345,9 @@ elseif (...)[2] then
 elseif (...)[3] then
    a = b()
    a.k3 = 3
+elseif (...)[4] then
+   a = b(1) or b(2)
+   a.k4 = 4
 else
    a = {}
    return a

src/luacheck/analyze.lua

@@ -180,7 +180,7 @@ local function is_function_var(var)
       #var.values == 2 and var.values[1].empty and var.values[2].type == "func")
 end
 
-local externally_accessible_tags = utils.array_to_set({"Id", "Index", "Call", "Invoke", "Dots"})
+local externally_accessible_tags = utils.array_to_set({"Id", "Index", "Call", "Invoke", "Op", "Paren", "Dots"})
 
 local function externally_accessible(value)
    return value.type ~= "var" or (value.node and externally_accessible_tags[value.node.tag])