When deducing an integral template argument for a non-type template

When deducing an integral template argument for a non-type template
parameter, keep the integral value exactly as it was in the source
code rather than trying to convert it to the type of the non-type
template parameter (which may still be dependent!). The value will
then be converted to the appropriate type once we check the resulting
template arguments. Fixes PR6707.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99611 91177308-0d34-0410-b5e6-96231b3b80d8