Add llvm::hexDigitValue to convert single characters to hex.

This is duplicated in a couple places in the codebase. Adopt this in APFloat.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172851 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/APFloat.cpp b/lib/Support/APFloat.cpp
index 4a7a5d1..2ac86a2 100644
--- a/lib/Support/APFloat.cpp
+++ b/lib/Support/APFloat.cpp
@@ -16,6 +16,7 @@
 #include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/FoldingSet.h"
 #include "llvm/ADT/Hashing.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
@@ -101,26 +102,6 @@
   return c - '0';
 }
 
-static unsigned int
-hexDigitValue(unsigned int c)
-{
-  unsigned int r;
-
-  r = c - '0';
-  if (r <= 9)
-    return r;
-
-  r = c - 'A';
-  if (r <= 5)
-    return r + 10;
-
-  r = c - 'a';
-  if (r <= 5)
-    return r + 10;
-
-  return -1U;
-}
-
 /* Return the value of a decimal exponent of the form
    [+-]ddddddd.