Completion does not work in attribute macros

[jonas-schievink]

During completion, we use this code to descend from a macro invocation to the expansion:

https://github.com/rust-analyzer/rust-analyzer/blob/1376ece497c2e189c97810e188e25c94f10951a8/crates/ide_completion/src/context.rs#L178-L182

This does not handle attribute macros, so completion doesn't work inside them. The code also makes use of speculative expansion, which is not yet implemented for attribute macros.