Support method names in portable PDBs

[loewenheim]

Currently, looking up an IL offset for a method in a PortablePdbCache file only returns line and file information:

pub struct LineInfo<'data> {
    /// The line in the source file.
    pub line: u32,
    /// The source file's name.
    pub file_name: &'data str,
    /// The source language.
    pub file_lang: Language,
}

However, the portable PDB file also contains the method name (and the type name of its receiver). These names are stored in the MethodDef and TypeDef tables, as detailed in sections II.22.26 and II.22.37, respectively, in the ECMA-335 standard.

We could parse these names out of the portable PDB file and add them to the cache format, similarly to how it works in symcache.