Change the name of the generator executables used to generate object code for the generator test cases under CMake.
Change the name of the generator executables used to generate
object code for the generator test cases under CMake.

The reason for doing this is the ``generator_`` prefix is confusing
(the test case executables start with the same prefix). We now follow
the Makefile build system convention where the generator executable has
a ``.generator`` suffix.

There are also a few tweaks to the comments to make things a little
clearer.


Former-commit-id: bd102f021a6f7860b46b08366e89067043afa264
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide