Add an explicit cast to pacify implicit boolean conversion warnings.

llvm-svn: 216539
diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
index 24b0e21..2e0519b 100644
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -663,7 +663,7 @@
 
 static const char *getDefaultTempDir(bool ErasedOnReboot) {
 #ifdef P_tmpdir
-  if (P_tmpdir)
+  if ((bool)P_tmpdir)
     return P_tmpdir;
 #endif