Recommit [C++20] [Modules] [ClangScanDeps] Allow clang-scan-deps to without...

Recommit [C++20] [Modules] [ClangScanDeps] Allow clang-scan-deps to without specified compilation database in P1689 (3/4)

In a private chat with @ben.boeckel , we get in consensus it would be
great for cmake if the invocation of clang-scan-deps can get rid of
compilation database. Due to the compilation database can't do very well
for the files which are not existed yet. @ben.boeckel may have more
context to add here.

This patch should be innocent for others usages.

Reviewed By: jansvoboda11

Differential Revision: https://reviews.llvm.org/D137534