Fix PR6159 and several other problems with value-dependent non-type template

arguments. This both prevents meaningless checks on these arguments and ensures
that they are represented as an expression by the instantiation.

Cleaned up and added standard text to the relevant test case. Also started
adding tests for *rejected* cases. At least one FIXME here where (I think) we
allow something we shouldn't. More to come in the area of rejecting crazy
arguments with decent diagnostics. Suggestions welcome for still better
diagnostics on these errors!

llvm-svn: 94953