Honor LINK_WHAT_YOU_USE when set to OFF

Explicitly setting variable CMAKE_LINK_WHAT_YOU_USE or property
LINK_WHAT_YOU_USE to OFF should not cause LWYU to run.  Fix the
property lookup to use GetPropertyAsBool.
1 job for link-what-you-use-property-as-bool
Status Name Job ID Coverage
  External
passed kwrobot-commit-check