Move TargetCache out of Transl:: and rm using directives of TargetCache

If we want to keep the namespace, we should make the naming
such that using directives aren't needed.  (Follow up diffs will make
the naming reasonable.)

Reviewed By: @edwinsmith

Differential Revision: D989131
Rename TargetCache:: to RDS::

In its own diff for reviewability.

Reviewed By: @edwinsmith

Differential Revision: D989132
Move runtime/vm/jit/target-cache.h to runtime/base/rds.h

Separate diff for rebase-ability and reviewability.

Reviewed By: @edwinsmith

Differential Revision: D989134
Cleaning up RDS, part 1

This is a quick start of cleaning up and documenting the RDS
(formerly target cache) module.  This diff contains:

  - a few renames to make things reasonable wrt the containing namespace

  - make Class::wouldCall a non-member function

  - various additional renames of "target cache" to "RDP"

  - the start of some documentation about what it is

Reviewed By: @markw65

Differential Revision: D989135
Remove various dead code relating to fb_rename_function

Apparently there used to be an API to limit the renamable
functions to a particular set, but this no longer exists.  Delete code
related to it.

Reviewed By: @bertmaher

Differential Revision: D989168
Remove FixedFuncCache, refactor LdFuncCache* instructions

FixedFuncCache was just one helper function which called
autoload.  The translations used offsetofs on FixedFuncCache even
though they were using allocFixedFunction to get the Handle now.  Move
the runtime helper out of the RDS namespace since it really didn't do
anything with RDS stuff (just calls loadFunc).  Changed LdFuncCached*
implementations to use extra-data.  Also I opted it back out of
native-calls, since it seems initially confusing to learn that it did
more than just a native call, and a cgCallHelper seemed clearer when
it does more than just a call: no need to go look up what it calls in
the other .cpp.

Reviewed By: @bertmaher

Differential Revision: D989157
ASAN build fixes for vixl

Never tried the vixl tests under ASAN before, this fixes a couple bugs.

Reviewed By: @edwinsmith

Differential Revision: D998518
Extend FBUnserializer API to allow it to be used by other algorithm

Allow FBUnserializer to be used as a building block for other algorithms using fb_serialization.

Reviewed By: zhuohuang

Differential Revision: D996689
Don't access uninitialized postorder ids in findDominator().

If a block isn't reachable, it will not be assigned a postorder
id in rpoSortBlocks().  The dominator algorithm incorrectly
used unreachable blocks postorder ids, which could lead to general
badness including crashes or miscompilations

Reviewed By: @jdelong

Differential Revision: D998809
Update README.md

Fix typos
Add CONTRIBUTING.md

Per https://github.com/blog/1184-contributing-guidelines, adding this
file will add a link at the top of issues and pull requests.
Merge pull request #1138 from elgenie/master

Add CONTRIBUTING.md