This got exposed when processing large LTO-generated files leading to
crashes.

On a few compilers (clang 11/12 for example [1]), the following does not
result in a copy elision, and since `Error`'s copy dtor is elided,
results in a compile error:

```
  Expect<Something> foobar() {
    ...
    if (Error E = aCallReturningError())
      return E;
    ...
  }
```

Moving `E` would, conversely, result in the pessimizing-move warning on
more recent clangs ("moving a local object in a return statement
prevents copy elision")

We just need to make the `Expected` ctor taking an `Error` take it as a
r-value reference.

[1] https://lab.llvm.org/buildbot/#/builders/54/builds/10505

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

Commit 4a794d848ce81f41c6e23a4d3d3f70c2d7d1baa4 caused build failure with -Werror -Wpessimizing-move on the clang-ppc64-aix buildbot. This patch applies Clang's suggestion to remove `std::move`.

Differential Revision: https://reviews.llvm.org/D119307

Instead of simply assuming that it will be zero. I double checked
that the bitstream reader doesn't have any special handling for
all-zero blobs, it will always write out the full contents.

If we're trying to reserve more memory than bits in the stream,
reject this early to avoid OOM.

Avoid report_fatal_error(), propagate the error upwards instead.

This code clearly intended to forward the error, but instead
returned the non-error result.