[skip ci] Do not cache tryCompiletimeConstantFold if isProvisional

Author: mbovel

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -4215,8 +4215,9 @@ object Types {
 
     def tryCompiletimeConstantFold(using Context): Type =
       if myTryCompiletimeConstantFoldPeriod != ctx.period then
-        myTryCompiletimeConstantFoldPeriod = ctx.period
         myTryCompiletimeConstantFold = tryCompiletimeConstantFoldImpl
+        if !isProvisional then
+          myTryCompiletimeConstantFoldPeriod = ctx.period
       myTryCompiletimeConstantFold.nn
 
     private def tryCompiletimeConstantFoldImpl(using Context): Type = tycon match {