[StaticAnalyzer] Fix false positives for vardecls that are technically...

[StaticAnalyzer] Fix false positives for vardecls that are technically unreachable but they are needed.

Example:

    switch (x) {
      int a;  // <- This is unreachable but needed
    case 1:
      a = ...

Differential Revision: https://reviews.llvm.org/D24905

llvm-svn: 282574