De-duplicate dependencies propagated through interface libraries

When processing an indirect dependency of target `A` on target `C` via
an interface library B, `cmComputeTargetDepends::AddTargetDepend` was
not checking for duplicate dependencies, so that if `A` depended on `C`
via multiple interface libraries, the dependency graph built in
cmComputeTargetDepends would have duplicate edges.  In a real project
(LLVM libc) this was causing cmake to consume multiple gigabytes of RAM.

Fixes: #27386