Warn about suspicious implicit conversions from floating point to bool

This warns in two specific situations:

1) For potentially swapped function arguments, e.g.

     void foo(bool, float);
     foo(1.7, false);

2) Misplaced brackets around function call arguments, e.g.

     bool InRange = fabs(a - b < delta);

   Where the last argument in a function call is implicitly converted
   from bool to float, and the function returns a float which gets
   implicitly converted to bool.

Patch by Andreas Eckleder!

llvm-svn: 162763