resolve: Don't speculatively load crates if this is a speculative resolution

This avoids a rare rustdoc bug where loading `core` twice caused a
'cannot find a built-in macro' error:

1. `x.py build --stage 1` builds the standard library and creates a sysroot
2. `cargo doc` does something like `cargo check` to create `rmeta`s for all the crates (unrelated to what was built above)
3. the `cargo check`-like `libcore-*.rmeta` is loaded as a transitive dependency *and claims ownership* of builtin macros
4. `rustdoc` later tries to resolve some path in a doc link
5. suggestion logic fires and loads "extern prelude" crates by name
6. the sysroot `libcore-*.rlib` is loaded and *fails to claim ownership* of builtin macros

This fixes step 5. by not running suggestion logic if this is a
speculative resolution. Additionally, it marks `resolve_ast_path` as a
speculative resolution.

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