[InlineCost] make InlineCost assignable

[fedor.sergeev]

Summary

Current InlineCost is not assignable because of const members Cost and Threshold.
I dont see practical benefits from having them const (access to these members is
private and internal interactions are rather simple). On other hand that makes
it hard to use as a member in some other data structure where assignability is necessary.

I'm going to use InlineCost in a downstream inliner that maintains a complex queue
of candidate call-sites and thus keeping and recalculating InlineCost is necessary.

This patch just removes 'const' from both members, making InlineCost assignable.

[apilipenko]

Even though it's motivated by downstream code, looks harmless to change upstream.

llvm/include/llvm/Analysis/InlineCost.h

 /// threshold for this cost metric.
 class InlineCost {
   enum SentinelValues {
     AlwaysInlineCost = INT_MIN,
     NeverInlineCost = INT_MAX
   };
 
   /// The estimated cost of inlining this callsite.
-  const int Cost;
+  int Cost;
 
   /// The adjusted threshold against which this cost was computed.
-  const int Threshold;
+  int Threshold;
 
   /// Must be set for Always and Never instances.
   const char *Reason = nullptr;
 
   // Trivial constructor, interesting logic in the factory functions below.
   InlineCost(int Cost, int Threshold, const char *Reason = nullptr)
       : Cost(Cost), Threshold(Threshold), Reason(Reason) {
     assert((isVariable() || Reason) &&