[analyzer] Fix the "Zombie Symbols" bug.

[analyzer] Fix the "Zombie Symbols" bug.

It's an old bug that consists in stale references to symbols remaining in the
GDM if they disappear from other program state sections as a result of any
operation that isn't the actual dead symbol collection. The most common example
here is:

   FILE *fp = fopen("myfile.txt", "w");
   fp = 0; // leak of file descriptor

In this example the leak were not detected previously because the symbol
disappears from the public part of the program state due to evaluating
the assignment. For that reason the checker never receives a notification
that the symbol is dead, and never reports a leak.

This patch not only causes leak false negatives, but also a number of other
problems, including false positives on some checkers.

What's worse, even though the program state contains a finite number of symbols,
the set of symbols that dies is potentially infinite. This means that is
impossible to compute the set of all dead symbols to pass off to the checkers
for cleaning up their part of the GDM.

No longer compute the dead set at all. Disallow iterating over dead symbols.
Disallow querying if any symbols are dead. Remove the API for marking symbols
as dead, as it is no longer necessary. Update checkers accordingly.

Differential Revision: https://reviews.llvm.org/D18860

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@347953 91177308-0d34-0410-b5e6-96231b3b80d8