Fix more -Wshorten-64-to-32 warnings.

llvm-svn: 50659
diff --git a/llvm/lib/System/Unix/Program.inc b/llvm/lib/System/Unix/Program.inc
index 91baefb..d0dade1 100644
--- a/llvm/lib/System/Unix/Program.inc
+++ b/llvm/lib/System/Unix/Program.inc
@@ -58,7 +58,7 @@
     return Path();
 
   // Now we have a colon separated list of directories to search; try them.
-  unsigned PathLen = strlen(PathStr);
+  size_t PathLen = strlen(PathStr);
   while (PathLen) {
     // Find the first colon...
     const char *Colon = std::find(PathStr, PathStr+PathLen, ':');