Revert "unbreak Modules/cxx20-export-import.cpp with LLVM_APPEND_VC_REV after 32b73bc"

This reverts commit 1689b10.

This patch was only added to fix a failure with 32b73bc, which
has been reverted again.

Author: fhahn

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 = 16;
+const unsigned VERSION_MAJOR = 15;
 
 /// AST file minor version number supported by this version of
 /// Clang.