STLExtras: Provide less/equal functors with templated function call operators,...

STLExtras: Provide less/equal functors with templated function call operators, plus a deref'ing functor template utility

Similar to the C++14 void specializations of these templates, useful as
a stop-gap until LLVM switches to '14.

Example use-cases in tblgen because I saw some functors that looked like
they could be simplified/refactored.

Reviewers: dexonsmith

Differential Revision: http://reviews.llvm.org/D7324

llvm-svn: 227828