Merge all the 'assignment' diagnostic code into one routine, decloning

it from several places.  This merges the diagnostics, making them more
uniform and fewer in number. This also simplifies and cleans up the code.

Some highlights:
1. This removes a bunch of very-similar diagnostics.
2. This renames AssignmentCheckResult -> AssignConvertType
3. This merges PointerFromInt + IntFromPointer which were always treated the same.
4. This updates a bunch of test cases that have minor changes to the produced diagnostics.

llvm-svn: 45589