rustc_codegen_llvm: Remove (almost) unused span parameter from many functions in metadata.rs

[michaelwoerister]

Many functions and intermediate data structures in rustc_codegen_llvm/src/debuginfo/metadata.rs take a span parameter that is only used for providing a span to a span_bug!() invocation, in case the debuginfo typemap gets corrupted. However, this span does not really convey useful information as it just points to the first point a type is used -- and half of the time is initialized to DUMMY_SP.

This PR removes this span parameter from the module.

It also removes the following unused parameters from composite_type_metadata() together with an outdated comment:

    // Ignore source location information as long as it
    // can't be reconstructed for non-local crates.
    _file_metadata: &'ll DIFile,
    _definition_span: Span,