Function and block layout checks. very basic CFG.

Function and block layout checks. very basic CFG.

This adds function and block layout checks to the validator. Very
basic CFG code has been added to make sure labels and branches
are correctly ordered.

Also:
* MemoryModel and Variable instruction checks/tests
* Use spvCheckReturn instead of CHECK_RESULT
* Fix invalid SSA tests
* Created libspirv::spvResultToString in diagnostic.h
* Documented various functions and classes
* Fixed error messages
* Fixed using declaration for FunctionDecl enum class