Introduced the notion of a "derived symbol" using the class SymbolDerived.

Introduced the notion of a "derived symbol" using the class SymbolDerived.
SymbolDerived allows us to model symbolic values that are related to other
symbols via a region hierarchy. For example, SymbolDerived can be used to model
individual values of a symbolic array.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@75728 91177308-0d34-0410-b5e6-96231b3b80d8