[APFloat] Fix checked error assert failures
APFLoat::convertFromString returns Expected result, which must be
"checked" if the LLVM_ENABLE_ABI_BREAKING_CHECKS preprocessor flag is
To mark an Expected result as "checked" we must consume the Error
In many cases, we are only interested in knowing if an error occured,
without the need to examine the error info. This is achieved, easily,
with the errorToBool() API.