[clang-tidy] Preserve comments and preprocessor directives when simplifying boolean expressions

This changeset still emits the diagnostic that the expression could be simplified, but it doesn't generate any fix-its that would lose comments or preprocessor directives within the text that would be replaced.

Fixes PR25842

Reviewers: alexfh

Subscribers: xazax.hun, cfe-commits

Patch by Richard Thomson! (+a naming style fix)

Differential Revision: http://reviews.llvm.org/D15737

llvm-svn: 256492