Changed CallRetValSymbol to SymbolConjured to allow "conjured" symbols to be...

Changed CallRetValSymbol to SymbolConjured to allow "conjured" symbols to be created for any expression, not just CallExprs.
Added experimental support for conjuring symbols during assingments where the RHS is "unknown".  This allows more value tracking for path-sensitivity.
Fixed bug in "assumption" logic when processing symbolic constraints; we would improperly mark constraints we didn't support as infeasible.

llvm-svn: 48306