Update compiler extension integration into the build system

The approach here is to create a new (empty) component, `Extensions', where all
statically compiled extensions dynamically register their dependencies. That way
we're more natively compatible with LLVMBuild and llvm-config.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=44870

Differential Revision: https://reviews.llvm.org/D78192

(cherry picked from commit 8f766e38)