Analyzer Core: Adding support for user-defined symbol dependencies. (For example...

Analyzer Core: Adding support for user-defined symbol dependencies. (For example, the allocated resource symbol only needs to be freed if no error has been returned by the allocator, so a checker might want to make the lifespan of the error code symbol depend on the allocated resource symbol.) Note, by default, the map that holds the dependencies will get destroyed along with the SymbolManager at the end of function exploration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137309 91177308-0d34-0410-b5e6-96231b3b80d8