[analyzer] Use isLiveRegion to determine when SymbolRegionValue is dead.

[analyzer] Use isLiveRegion to determine when SymbolRegionValue is dead.

Fixes a FIXME, improves dead symbol collection, suppresses a false positive,
which resulted from reusing the same symbol twice for simulation of 2 calls to the same function.

Fixing this lead to 2 possible false negatives in CString checker. Since the checker is still alpha and
the solution will not require revert of this commit, move the tests to a FIXME section.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177206 91177308-0d34-0410-b5e6-96231b3b80d8