[Analysis] Make members of InlineCost const (NFC)

[kazu]

Summary

Once we create an instance of InlineCost, we don't change its
contents.

[yrouban]

It is very inconvenient to have non-static const fields.
In our downstream source we have a field of InlineCost that is changing. With this patch we cannot change the value of the field. The same problem is with locals of InlineCost type.
Consider reverting this patch, please.

[kazu]

In D134388#3811157, @yrouban wrote:

It is very inconvenient to have non-static const fields.
In our downstream source we have a field of InlineCost that is changing. With this patch we cannot change the value of the field. The same problem is with locals of InlineCost type.
Consider reverting this patch, please.

Sure. I just reverted the patch.

llvm/include/llvm/Analysis/InlineCost.h

 /// Objects of this type also provide the adjusted threshold for inlining
 /// based on the information available for a particular callsite. They can be
 /// directly tested to determine if inlining should occur given the cost and
 /// threshold for this cost metric.
 class InlineCost {
   enum SentinelValues { AlwaysInlineCost = INT_MIN, NeverInlineCost = INT_MAX };
 
   /// The estimated cost of inlining this callsite.
-  int Cost = 0;
+  const int Cost = 0;
 
   /// The adjusted threshold against which this cost was computed.
-  int Threshold = 0;
+  const int Threshold = 0;
 
   /// Must be set for Always and Never instances.
-  const char *Reason = nullptr;
+  const char *const Reason = nullptr;
 
   /// The cost-benefit pair computed by cost-benefit analysis.
-  Optional<CostBenefitPair> CostBenefit = None;
+  const Optional<CostBenefitPair> CostBenefit = None;
 
   // Trivial constructor, interesting logic in the factory functions below.
   InlineCost(int Cost, int Threshold, const char *Reason = nullptr,
              Optional<CostBenefitPair> CostBenefit = None)
       : Cost(Cost), Threshold(Threshold), Reason(Reason),
         CostBenefit(CostBenefit) {
     assert((isVariable() || Reason) &&
            "Reason must be provided for Never or Always");