Refactor: Simplify boolean expressions in llvm Support

Simplify boolean expressions using `true` and `false` with `clang-tidy`

Patch by Richard Thomson - I dropped the parens and != 0 test, for
consistency with other patches/tests like this, but I'm open to the
notion that we should add the explicit non-zero test in all these sort
of cases (non-bool assigned to a bool).

Differential Revision: http://reviews.llvm.org/D8526

llvm-svn: 233004