[analyzer] DeadStoresChecker: Treat locals captured by reference in C++ lambdas as escaped.

The analyzer currently reports dead store false positives when a local variable
is captured by reference in a C++ lambda.

For example:

  int local = 0; auto lambda = [&local]() {
    local++;
  };
  local = 7; // False Positive: Value stored to 'local' is never read
  lambda();

In this case, the assignment setting `local` to 7 is not a dead store because
the called lambda will later read that assigned value.

This commit silences this source of false positives by treating locals captured
by reference in C++ lambdas as escaped, similarly to how the DeadStoresChecker
deals with locals whose address is taken.

rdar://problem/22165179

llvm-svn: 253630