When checking the template argument list, use a copy of that list for changes

and only update the orginal list on a valid arugment list.  When checking an
individual expression template argument, and conversions are required, update
the expression in the template argument.  Since template arguments are
speculatively checked, the copying of the template argument list prevents
updating the template arguments when the list does not match the template.

Additionally, clean up the integer checking code in the template diffing code.
The code performs unneccessary conversions from APSInt to APInt.

Fixes PR21758.

This essentially reverts r224770 to recommits r224667 and r224668 with extra
changes to prevent the template instantiation problems seen in PR22006.
A test to catch the discovered problem is also added.

llvm-svn: 226983