This is the first step towards hiding backend-specific stuff from the
rest of subzero. In this CL, all the references to target-specific files
(e.g., IceTargetLoweringX8632.h) are removed from target-independent
files.

This CL also changes the named constructors in the Target-specific
classes (e.g., TargetX8632::create()) to return unique_ptrs.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1551633002 .