[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



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282574 91177308-0d34-0410-b5e6-96231b3b80d8