Reserve unused bits in struct CXIndexOptions; NFC

[aaron.ballman]

LGTM!

clang/include/clang-c/Index.h

typedef struct CXIndexOptions {
   /**
    * \see clang_createIndex()
    */
   unsigned ExcludeDeclarationsFromPCH : 1;
   /**
    * \see clang_createIndex()
    */
   unsigned DisplayDiagnostics : 1;
+  unsigned /*Reserved*/ : 14;
+
   /**
    * The path to a directory, in which to store temporary PCH files. If null or
    * empty, the default system temporary directory is used. These PCH files are
    * deleted on clean exit but stay on disk if the program crashes or is killed.
    *
    * Libclang does not create the directory at the specified path in the file
    * system. Therefore it must exist, or storing PCH files will fail.
    */

clang/tools/libclang/CIndex.cpp

CXIndex clang_createIndexWithOptions(const CXIndexOptions *options) {
   // sizeof(CXIndexOptions) must increase for versioning purposes.
   // 2. Options should be added at the end of the struct in order to seamlessly
   // support older struct versions. If options->Size < sizeof(CXIndexOptions),
   // don't attempt to read the missing options and rely on the default values of
   // recently added options being reasonable. For example:
   // if (options->Size >= offsetof(CXIndexOptions, RecentlyAddedMember))
   //   do_something(options->RecentlyAddedMember);
 
+  // An exception: if a new option is small enough, it can be squeezed into the
+  // /*Reserved*/ bits in CXIndexOptions. Since the default value of each option
+  // is guaranteed to be 0 and the callers are advised to zero out the struct,
+  // programs built against older libclang versions would implicitly set the new
+  // options to default values, which should keep the behavior of previous
+  // libclang versions and thus be backward-compatible.
+
   // If options->Size > sizeof(CXIndexOptions), the user may have set an option
   // we can't handle, in which case we return nullptr to report failure.
   // Replace `!=` with `>` here to support older struct versions. `!=` has the
   // advantage of catching more usage bugs and no disadvantages while there is a
   // single supported struct version (the initial version).
   if (options->Size != sizeof(CXIndexOptions))
     return nullptr;
   CIndexer *const CIdxr = clang_createIndex_Impl(