Make clang_Cursor_getMangling not mangle if the declaration isn't mangled

Right now clang_Cursor_getMangling will attempt to mangle any
declaration, even if the declaration isn't mangled (extern C).  This
results in a partially mangled name which isn't useful for much. This
patch makes clang_Cursor_getMangling return an empty string if the
declaration isn't mangled.

Patch by Michael Wu <mwu@mozilla.com>.

llvm-svn: 253909
diff --git a/clang/include/clang-c/Index.h b/clang/include/clang-c/Index.h
index 97d1dde..b831481 100644
--- a/clang/include/clang-c/Index.h
+++ b/clang/include/clang-c/Index.h
@@ -2461,6 +2461,32 @@
 CINDEX_LINKAGE enum CXLinkageKind clang_getCursorLinkage(CXCursor cursor);
 
 /**
+ * \brief Describe the visibility of the entity referred to by a cursor.
+ *
+ * This returns the default visibility if not explicitly specified by
+ * a visibility attribute. The default visibility may be changed by
+ * commandline arguments.
+ *
+ * \param cursor The cursor to query.
+ *
+ * \returns The visibility of the cursor.
+ */
+enum CXVisibilityKind {
+  /** \brief This value indicates that no visibility information is available
+   * for a provided CXCursor. */
+  CXVisibility_Invalid,
+
+  /** \brief Symbol not seen by the linker. */
+  CXVisibility_Hidden,
+  /** \brief Symbol seen by the linker but resolves to a symbol inside this object. */
+  CXVisibility_Protected,
+  /** \brief Symbol seen by the linker and acts like a normal symbol. */
+  CXVisibility_Default,
+};
+
+CINDEX_LINKAGE enum CXVisibilityKind clang_getCursorVisibility(CXCursor cursor);
+
+/**
  * \brief Determine the availability of the entity that this cursor refers to,
  * taking the current target platform into account.
  *