Fixed a crash in the integrated assembler for Mach-O when a symbol difference
expression uses an assembler temporary symbol from an assignment.  In this case
the symbol does not have a fragment so the use of getFragment() would be NULL
and caused a crash. In the case of an assembler temporary symbol we want to use
the AliasedSymbol (if any) which will create a local relocation entry, but if
it is not an assembler temporary symbol then let it use that symbol with an
external relocation entry.

rdar://9356266