Fix a typo in the function name that I just introduced (r201155)

llvm-svn: 201156
diff --git a/clang/tools/libclang/Indexing.cpp b/clang/tools/libclang/Indexing.cpp
index 7fb27bf..dc7a0c0 100644
--- a/clang/tools/libclang/Indexing.cpp
+++ b/clang/tools/libclang/Indexing.cpp
@@ -756,7 +756,7 @@
   unsigned index_options = ITUI->index_options;
   ITUI->result = 1; // init as error.
 
-  if (isNotUseableTU(TU)) {
+  if (isNotUsableTU(TU)) {
     LOG_BAD_TU(TU);
     return;
   }