Implement lazy "copying" of structures and arrays in RegionStore. While

RegionStore already lazily abstracted the contents of arrays and structs, when
doing an assignment from one array/struct to another we did an explicit
element-wise copy, which resulted in a loss of laziness and huge performance
problem when analyzing many code bases.

Now RegionStoreManager handles such assignments using a new SVal could
'LazyCompoundSVal', which basically means the value of a given struct or array
(a MemRegion*) in a specific state (GRState). When we do a load from a field
whose encompassing struct binds to a LazyCompoundSVal, we essentially do a field
lookup in the original structure. This means we have essentially zero copying of
data for structs/arrays and everything stays lazy.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@78268 91177308-0d34-0410-b5e6-96231b3b80d8