[analyzer] Remove the "postponed" hack, deal with derived symbols using an extra map

The "derived" symbols indicate children fields of a larger symbol.
As parents do not have pointers to their children, the garbage
collection algorithm the analyzer currently uses adds such symbols into
a "postponed" category, and then keeps running through the worklist
until the fixed point is reached.

The current patch rectifies that by instead using a helper map which
stores pointers from parents to children, so that no fixed point
calculation is necessary.

The current patch yields ~5% improvement in running time on sqlite.

Differential Revision: https://reviews.llvm.org/D51397

llvm-svn: 341722