Fix after r315079

Microsoft's debug implementation of std::copy checks if the destination is an
array and then does some bounds checking.  This was causing an assertion
failure in fs::rename_internal which copies to a buffer of the appropriate
size but that's type-punned to an array of length 1 for API compatibility
reasons.

Fix is to make make the destination a pointer rather than an array.

llvm-svn: 315222