[patch] Use a more idiomatic bool operator in ErrorOr

[Rafael Espíndola]

> Do we have types with any such dangerous duplicate conversions? I'd say
> that's likely bad design even in a world with explicit bool conversion & I'd
> treat those as code/design bugs and on that basis I'd still prefer to nuke
> safe bools and use LLVM_EXPLICIT entirely.

Same here. We already use LLVM_EXPLICIT in OwningPtr.h. IMHO MSVC 2012
is not a big enough reason to avoid it. We have enough compilers that
do support it to avoid bad bool conversions showing up in the
codebase.

Cheers,
Rafael