Change default error_code ctor to a 'named ctor' so it's more self-documenting.
Unify default construction of error_code uses on this idiom so that users don't
feel compelled to make static globals for naming convenience. (unfortunately I
couldn't make the original ctor private as some APIs don't return their result,
instead using an out parameter (that makes sense to default construct) - which
is a bit of a pity. I did, however, find/fix some cases of unnecessary default
construction of error_code before I hit the unfixable cases)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150197
91177308-0d34-0410-b5e6-
96231b3b80d8