Rather than reimplement, use a `using` declaration to bring in
`SmallVectorImpl<char>`'s assign and append implementations in
`SmallString`.
The `SmallString` versions were missing reference invalidation
assertions from `SmallVector`. This patch also fixes a bug in
`llvm::FileCollector::addFileImpl`, which was a copy/paste from
`clang::ModuleDependencyCollector::copyToRoot`, both caught by the
no-longer-skipped assertions.
As a drive-by, this also sinks the `const SmallVectorImpl&` versions of
these methods down into `SmallVectorImpl`, since I imagine they'd be
useful elsewhere.
Differential Revision: https://reviews.llvm.org/D95202
e675c8ca11