[CMake] Use .svn/wc.db as our VC dependency if it's available.

In order to keep SVNVersion.inc from being regenerated on every build, we need
to specify as a dependency a file that only changes when the VC state updates.
I previously just had .svn/entries, but that's only used by Subversion 1.6.
1.7 uses .svn/wc.db instead; prefer that if it's present.

(Thanks, Nico!)

llvm-svn: 222407