[clang] unbreak Index/preamble-reparse-changed-module.m with LLVM_APPEND_VC_REV=NO after 46d4d1fea4

See discussion starting at https://reviews.llvm.org/D96816#2572431 .
The same thing is happening with 46d4d1fea4.

clang/include/clang/Serialization/ASTBitCodes.h

@@ -41,7 +41,7 @@ namespace serialization {
 /// Version 4 of AST files also requires that the version control branch and
 /// revision match exactly, since there is no backward compatibility of
 /// AST files at this time.
-const unsigned VERSION_MAJOR = 12;
+const unsigned VERSION_MAJOR = 13;
 
 /// AST file minor version number supported by this version of
 /// Clang.