Be a bit more consistent about using ErrorOr when constructing Binary objects.

The constructors of classes deriving from Binary normally take an error_code
as an argument to the constructor. My original intent was to change them
to have a trivial constructor and move the initial parsing logic to a static
method returning an ErrorOr. I changed my mind because:

* A constructor with an error_code out parameter is extremely convenient from
  the implementation side. We can incrementally construct the object and give
  up when we find an error.
* It is very efficient when constructing on the stack or when there is no
  error. The only inefficient case is where heap allocating and an error is
  found (we have to free the memory).

The result is that this is a much smaller patch. It just standardizes the
create* helpers to return an ErrorOr.

Almost no functionality change: The only difference is that this found that
we were trying to read past the end of COFF import library but ignoring the
error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199770 91177308-0d34-0410-b5e6-96231b3b80d8
[SROA] Fix a bug which could cause the common type finding to return
inconsistent results for different orderings of alloca slices. The
fundamental issue is that it is just always a mistake to return early
from this function. There is no effective early exit to leverage. This
patch stops trynig to do so and simplifies the code a bit as
a consequence.

Original diagnosis and patch by James Molloy with some name tweaks by me
in part reflecting feedback from Duncan Smith on the mailing list.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199771 91177308-0d34-0410-b5e6-96231b3b80d8