Correctly treat 64 bit integers specified via the mode attribute as the 'long'

type when that type is 64 bits wide, and the 'long long' type when 'long' is
only 32 bits wide. This fixes PR6108.

Also adds a bunch of test cases to ensure proper conversion between equally
sized standard types and mode-specified types on both 32 and 64 bit targets.

llvm-svn: 94527