[ObjCARC Annotations] Implemented ARC annotation metadata to expose the ARC data flow analysis state in the IR via metadata.

Previously the inner works of the data flow analysis in ObjCARCOpts was hard to
get out of the optimizer for analysis of bugs or testing. All of the current ARC
unit tests are based off of testing the effect of the data flow
analysis (i.e. what statements are removed or moved, etc.). This creates
weakness in the current unit testing regimem since we are not actually testing
what effects various instructions have on the modeled pointer state.
Additionally in order to analyze a bug in the optimizer, one would need to track
by hand what the optimizer was actually doing either through use of DEBUG
statements or through the usage of a debugger, both yielding large loses in
developer productivity.

This patch deals with these two issues by providing ARC annotation
metadata that annotates instructions with the state changes that they cause in
various pointers as well as provides metadata to annotate provenance sources.

Specifically, we introduce the following metadata types:

1. llvm.arc.annotation.bottomup.
2. llvm.arc.annotation.topdown.
3. llvm.arc.annotation.provenancesource.

llvm.arc.annotation.{bottomup,topdown}: These annotations describes a state
change in a pointer when we are visiting instructions bottomup/topdown
respectively. The output format for both is the same:

  !1 = metadata !{metadata !"(test,%x)", metadata !"S_Release", metadata !"S_Use"}

The first element is a string tuple with the following format:

  (function,variable name)

The second two elements of the metadata show the previous state of the
pointer (in this case S_Release) and the new state of the pointer (S_Use). We
write the metadata in such a manner to ensure that it is easy for outside tools
to parse. This is important since I am currently working on a tool for taking
this information and pretty printing it besides the IR and that can be used for
LIT style testing via the generation of an index.

llvm.arc.annotation.provenancesource: This metadata is used to annotate
instructions which act as provenance sources, i.e. ones that introduce a
new (from the optimizer's perspective) non-argument pointer to track. This
enables cross-referencing in between provenance sources and the state changes
that occur to them.

This is still a work in progress. Additionally I plan on committing
later today additions to the annotations that annotate at the top/bottom
of basic blocks the state of the various pointers being tracked.

*NOTE* The metadata support is conditionally compiled into libObjCARCOpts only
when we are producing a debug build of llvm/clang and even so are
disabled by default. To enable the annotation metadata, pass in
-enable-objc-arc-annotations to opt.