[CMake] Default to static linking for subprojects.

Pass plugins introduced in D61446 do not support dynamic linking on
Windows, hence the option LLVM_${name_upper}_LINK_INTO_TOOLS can only
work being set to "ON". Currently, it defaults to "OFF" such that such
plugins are inoperable by default on Windows. Change the default for
subprojects to follow LLVM_ENABLE_PROJECTS.

Reviewed By: serge-sans-paille, MaskRay

Differential Revision: https://reviews.llvm.org/D72372