Fix a use-after-free in `llvm-config`.

Summary:
This could happen if `GetComponentNames` is true, because `Name` from
`VisitComponent` would reference a stack instance of `std::string` in
`ComputeLibsForComponents`.

Reviewers: beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14913

llvm-svn: 254108