Fix PR15634, better error message for template deduction failure.

When two template decls with the same name are used in this diagnostic,
force them to print their qualified names.  This changes the bad message of:

candidate template ignored: could not match 'array' against 'array'

to the better message of:

candidate template ignored: could not match 'NS2::array' against 'NS1::array'

llvm-svn: 179056