Revert "[analyzer] Remove the "postponed" hack, deal with derived symbols..."

Revert "[analyzer] Remove the "postponed" hack, deal with derived symbols..."

This reverts commit r341722.

The "postponed" mechanism turns out to be necessary in order to handle
situations when a symbolic region is only kept alive by implicit bindings
in the Store. Otherwise the region is never scanned by the Store's worklist
and the binding gets dropped despite being live, as demonstrated
by the newly added tests.

Differential Revision: https://reviews.llvm.org/D57554

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@353350 91177308-0d34-0410-b5e6-96231b3b80d8