unbreak Modules/cxx20-export-import.cpp with LLVM_APPEND_VC_REV after fa34951fbc

See revision b8b7a9dcdc for prior art.

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 = 17;
+const unsigned VERSION_MAJOR = 18;
 
 /// AST file minor version number supported by this version of
 /// Clang.