Add AST-based fallback path for symbols

Symbol search is functionality which allows you to enter query like Aex and get fuzzy search results for all symbols with matching name (like AstExpr). Originally, it was syntax based, but that was missing symbols declared by macros. In https://github.com/rust-lang/rust-analyzer/pull/10872 we rewrote the infra to be based on semantics. This generally works much better, but has one drawback: if we don't have semantic information for a file, we don't get symbols for it.

There are three cases where we lack semantics:

• when file is not a part of the module tree
• when the project model is broken (typo in Cargo.toml)
• when we are just loading the project

For all three cases, it would be much better if we fell back on the older, syntax based infra.

I think the logic we want here is:

• collect all semantic modules
• collect all semantic symbols from modules
• collect all files (which doesn't require semantic info)
• key idea: for each file, which isn't a file of some module, collect syntactic symbols

Start by reading the semantic and syntax-based symbol-search infrastructure described in rust-analyzer#10872, then trace how semantic modules and files are collected. Implement the fallback for files that are not part of a semantic module, and verify that semantic symbols remain unchanged while those files contribute syntactic symbols.