Silence -Wsign-compare warnings from GCC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140043 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/APInt.cpp b/lib/Support/APInt.cpp
index 3cb7576..d7291a1 100644
--- a/lib/Support/APInt.cpp
+++ b/lib/Support/APInt.cpp
@@ -54,11 +54,11 @@
       return r;
 
     r = cdigit - 'A';
-    if (r <= radix-11)
+    if (r <= radix - 11U)
       return r + 10;
 
     r = cdigit - 'a';
-    if (r <= radix-11)
+    if (r <= radix - 11U)
       return r + 10;
   }