[CMake] Be more consistent about naming targets and components

Summary:
The point of this patch is to have a consistent convention for naming build, check and install targets so that the targets can be constructed from the project name.

This change renames a bunch of CMake components and targets from libcxx to cxx. For each renamed target I've added a convenience target that matches the old target name and depends on the new target. This will preserve function of the old targets so that the change doesn't break the world. We can evaluate if it is worth removing the extra targets later.

Reviewers: EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23699

llvm-svn: 279675