DebugInfo: Cleanup testing of non-type template parameters.

Separate out the non-nullable parameters from the nullable ones
(currently only the template template parameter) and demonstrate that
cv-qualifiers aren't preserved for non-null parameters (but are
preserved for null parameters) by adding 'const' to an int* non-type
template parameter.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219883 91177308-0d34-0410-b5e6-96231b3b80d8