[clang-tidy] Add a checker for implicit bool conversion of a bool*.

The goal is to find code like the example below, which is likely a typo
where someone meant to write "if (*b)".
bool *b = SomeFunction();
 if (b) {
   // b never dereferenced
 }

This checker naturally has a relatively high false positive rate so it
applies some heuristics to avoid cases where the pointer is checked for
nullptr before being written.

Differential Revision: http://reviews.llvm.org/D4458