runtime: namespace SmallVectorImpl for inline namespace

[compnerd]

This adjusts the use of SmallVectorImpl to allow the runtime to use
inline namespaces for its local copy of LLVMSupport.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

CC: @mikeash @jckarter

[compnerd]

@swift-ci please test

[jckarter]

Instead of using an inline namespace, can we alter the Swift versions of these templates to live in the swift namespace rather than llvm? I think it's better to make it clear we're using vendored versions of these templates than play tricks to make it look like they're the same as the original LLVM declarations.

[compnerd]

The inline namespace is a prefix of __swift::__runtime::. So the final qualified name is __swift::__runtime::llvm::ArrayRef etc. The reason for the inline namespace is to avoid change all the sites. This becomes more useful for the shared headers across the compiler and the runtime, where the namespace is going to remain llvm:: (unless we completely fork LLVM).

[compnerd]

Going to merge this for now; if I've misunderstood what you were recommending, I can do the changes in a follow up.

[jckarter]

"Change all the sites" is what I was trying to suggest. How prevalent do the affected types show up in shared runtime/compiler code? I'd be concerned about divergent behavior between Swift and LLVM causing really subtle hard-to-track-down problems. When we've vendored LLVM templates previously like TrailingObjects, we've intentionally re-namespaced them to make the distinction clear.