[PDB] Fix failure on big endian machines.

We changed an ArrayRef<uint8_t> to an ArrayRef<uint32_t>, but
it needs to be an ArrayRef<support::ulittle32_t>.

We also change ArrayRef<> to FixedStreamArray<>.  Technically
an ArrayRef<> will work, but it can cause a copy in the underlying
implementation if the memory is not contiguous, and there's no
reason not to use a FixedStreamArray<>.

Thanks to nemanjai@ and thakis@ for helping me track this down
and confirm the fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344063 91177308-0d34-0410-b5e6-96231b3b80d8