Support: ensure proper state in ErrorOr copy ctors before calling 'get'

Some paths through the copy constructors for 'ErrorOr' were calling
'get' when 'HasError' and 'IsValid' were not properly initialized.
Depending on what happened to be in memory for those member variables
the asserts in 'get' might incorrectly fire.  Fixed by ensuring that
the member variables in question are always initialized before calling
'get'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174381 91177308-0d34-0410-b5e6-96231b3b80d8