[tablegen] !strconcat accepts more than two arguments but this wasn't documented or tested.

Summary:
* Updated the documentation
* Added a test for >2 arguments
* Added a check for the lexical concatenation
* Made the existing test a bit stricter.

Reviewers: t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, llvm-commits

Differential Revision: http://reviews.llvm.org/D3485