[clang-tidy] Add readability-simplify-boolean-expr check to clang-tidy

This check looks for comparisons between boolean expressions and boolean
constants and simplifies them to just use the appropriate boolean expression
directly.

if (b == true) becomes if (b)
if (b == false) becomes if (!b)
if (b && true) becomes if (b)
if (b && false) becomes if (false)
if (b || true) becomes if (true)
if (b || false) becomes if (b)
e ? true : false becomes e
e ? false : true becomes !e
if (true) t(); else f(); becomes t();
if (false) t(); else f(); becomes f();
if (e) return true; else return false; becomes return (e);
if (e) return false; else return true; becomes return !(e);
if (e) b = true; else b = false; becomes b = e;
if (e) b = false; else b = true; becomes b = !(e);

http://reviews.llvm.org/D7648

Patch by Richard Thomson!

llvm-svn: 234626