Given that we are removing codegen related files, we need to clean up the build targets in the current build file. There will be a series of PRs for this.

The new build targets should roughly follow this dependency graph.

┌───────┐┌───────────────┐        
│grpc++ ││grpc++_unsecure│        
└┬─────┬┘└┬─────────┬────┘        
┌▽───┐┌▽──▽───────┐┌▽────────────┐
│grpc││grpc++_base││grpc_unsecure│
└┬───┘└┬──────────┘└┬────────────┘
┌▽─────▽────────────▽┐            
│grpc_base           │            
└────────────────────┘            

Detailed steps is the following.

• (Covered in this PR) gpr_base is merged into gpr. The gpr here itself is nothing but depending on gpr_base. I don't think we need to separate them.
• grpc++_internals and grpc++_internal_hdrs_only are merged into grpc++. The reason is similar. grpc++ does not contain much stuff but grpc++_internals.
• (Covered in this PR) gpr_codegen is merged to gpr since the files in gpr_codegen should be removed in the future and be moved to gpr.
• grpc_codegen is merged to grpc_base with the similar reason as above.
• grpc++_codegen_base and grpc++_codegen_base_src are merged to grpc++_base with the similar reason as above.
• grpc_secure is merged to grpc since grpc is by default secure. We do not need to separate them.
• grpc++_base_unsecure is merged either into grpc++_base or into grpc++_unsecure.

This PR removes gpr_base and merges gpr_codegen into gpr without removing gpr_codegen for temporary compatibility. It should be removed in the following PRs.