CMake: try to actually fix the regexes for setting HOST_LINK_VERSION this time (PR21268)

The regex should not accept an empty version number.

The previous attempt at r223106 failed, and the build was still erroring:

  CMake Error at tools/clang/CMakeLists.txt:269 (string):
    string sub-command REGEX, mode REPLACE regex "[^0-9]*([0-9.]*).*" matched
    an empty string.

llvm-svn: 223333