The first problem to fix is to stop creating synthetic *Table_gen
targets next to all of the LLVM libraries. These had no real effect as
CMake specifies that add_custom_command(OUTPUT ...) directives (what the
'tablegen(...)' stuff expands to) are implicitly added as dependencies
to all the rules in that CMakeLists.txt.

These synthetic rules started to cause problems as we started more and
more heavily using tablegen files from *subdirectories* of the one where
they were generated. Within those directories, the set of tablegen
outputs was still available and so these synthetic rules added them as
dependencies of those subdirectories. However, they were no longer
properly associated with the custom command to generate them. Most of
the time this "just worked" because something would get to the parent
directory first, and run tablegen there. Once run, the files existed and
the build proceeded happily. However, as more and more subdirectories
have started using this, the probability of this failing to happen has
increased. Recently with the MC refactorings, it became quite common for
me when touching a large enough number of targets.

To add insult to injury, several of the backends *tried* to fix this by
adding explicit dependencies back to the parent directory's tablegen
rules, but those dependencies didn't work as expected -- they weren't
forming a linear chain, they were adding another thread in the race.

This patch removes these synthetic rules completely, and adds a much
simpler function to declare explicitly that a collection of tablegen'ed
files are referenced by other libraries. From that, we can add explicit
dependencies from the smaller libraries (such as every architectures
Desc library) on this and correctly form a linear sequence. All of the
backends are updated to use it, sometimes replacing the existing attempt
at adding a dependency, sometimes adding a previously missing dependency
edge.

Please let me know if this causes any problems, but it fixes a rather
persistent and problematic source of build flakiness on our end.

llvm-svn: 136023