Collect IRExecutionUnits as part of persistent expression state.

IRExecutionUnits contain code and data that persistent declarations can
depend on.  In order to keep them alive and provide for lookup of these
symbols, we now allow any PersistentExpressionState to keep a list of
execution units.  Then, when doing symbol lookup on behalf of an 
expression, any IRExecutionUnit can consult the persistent expression
states on a particular Target to find the appropriate symbol.

<rdar://problem/22864976>

llvm-svn: 263995