[analyzer] Fix symbolication for unknown unary increment/decrement results.

If the value is known, but we cannot increment it, conjure a symbol to
represent the result of the operation based on the operator expression,
not on the sub-expression.

In particular, no longer crash on comparing a result of a LocAsInteger increment
to a constant integer.

rdar://problem/31067356

Differential Revision: https://reviews.llvm.org/D31289

llvm-svn: 298927