[memprof] Fix a warning

This patch fixes:

  llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp:4771:9:
  error: non-void lambda does not return a value in all control paths
  [-Werror,-Wreturn-type]

Author: kazutakahirata

llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp

@@ -4766,8 +4766,8 @@ bool CallsiteContextGraph<DerivedCCG, FuncTy, CallTy>::assignFunctions() {
             if (!FuncCloneToCurNodeCloneMap.count(CF.FuncClone))
               return CF.FuncClone;
           }
-          assert(false &&
-                 "Expected an available func clone for this callsite clone");
+          llvm_unreachable(
+              "Expected an available func clone for this callsite clone");
         };
 
         // See if we can use existing function clone. Walk through