make APFloat::toString be const.

llvm-svn: 97883
diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 619f061..8f860a6 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -3366,7 +3366,7 @@
 
 void APFloat::toString(SmallVectorImpl<char> &Str,
                        unsigned FormatPrecision,
-                       unsigned FormatMaxPadding) {
+                       unsigned FormatMaxPadding) const {
   switch (category) {
   case fcInfinity:
     if (isNegative())