SILOptimizer: use LLVM_ATTRIBUTE_USED

This makes the use of the used attribute more portable.
`__attribute__((__used__))` is not accepted by Visual Studio, but LLVM
conveniently provides the helpful macro `LLVM_ATTRIBUTE_USED` to enable this.

Author: compnerd

lib/SILOptimizer/Mandatory/PredictableMemOpt.cpp

@@ -247,7 +247,7 @@ struct AvailableValue {
     return {NewValue, SubElementNumber, InsertionPoints};
   }
 
-  void dump() const __attribute__((used));
+  void dump() const LLVM_ATTRIBUTE_USED;
   void print(llvm::raw_ostream &os) const;
 
 private:
@@ -379,7 +379,7 @@ class AvailableValueAggregator {
   SILValue aggregateValues(SILType LoadTy, SILValue Address, unsigned FirstElt);
 
   void print(llvm::raw_ostream &os) const;
-  void dump() const __attribute__((used));
+  void dump() const LLVM_ATTRIBUTE_USED;
 
 private:
   SILValue aggregateFullyAvailableValue(SILType LoadTy, unsigned FirstElt);