[Analysis] Fix -Wunused-function in ConstantFolding.cpp (NFC)

/llvm-project/llvm/lib/Analysis/ConstantFolding.cpp:1747:11:
error: unused function 'GetConstantFoldFPValue128' [-Werror,-Wunused-function]
Constant *GetConstantFoldFPValue128(float128 V, Type *Ty) {
          ^
1 error generated.

llvm/lib/Analysis/ConstantFolding.cpp

@@ -1743,7 +1743,7 @@ Constant *GetConstantFoldFPValue(double V, Type *Ty) {
   llvm_unreachable("Can only constant fold half/float/double");
 }
 
-#if defined(HAS_IEE754_FLOAT128)
+#if defined(HAS_IEE754_FLOAT128) && defined(HAS_LOGF128)
 Constant *GetConstantFoldFPValue128(float128 V, Type *Ty) {
   if (Ty->isFP128Ty())
     return ConstantFP::get(Ty, V);