Looks for duplicate includes and removes them.

Every time an include directive is processed, check a vector of filenames
to see if the included file has already been included.  If so, it issues
a warning and a replacement to remove the entire line containing the
duplicated include directive.

When a macro is defined or undefined, the vector of filenames is cleared.
This enables including the same file multiple times, but getting
different expansions based on the set of active macros at the time of
inclusion.  For example:

  #undef NDEBUG
  #include "assertion.h"
  // ...code with assertions enabled

  #define NDEBUG
  #include "assertion.h"
  // ...code with assertions disabled

Since macros are redefined between the inclusion of assertion.h,
they are not flagged as redundant.

Differential Revision: https://reviews.llvm.org/D7982