If this flag is set, (*Config).Load will not return an
error even if some packages had type errors. Each individual
PackageInfo can be queried for its error state, now exposed as
TypeError.
In addition, each PackageInfo exposes whether it is
"transitively error-free". ssa.Create skips packages without
this flag since it is required for SSA construction.
+ Test.
LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/62000045
This results in significant improvement to type-checking time:
it reduces by 4% the entire running time of ssa/stdlib_test
(GOMAXPROCS=8, n=7).
LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/57770043