[JuliaLowering] Reconstruct macro name before, not during, macro expansion

[mlechu]

On top of #60162. This simplification lets macro expansion handle the macro name as an identifier rather than the nonterminal K"macro_name" node. That and the old MacroName node (as well as CmdMacroName, StrMacroName) are mainly the result of restrictions on raw parser output: we can't parse @Base.x to a non-contiguous @x, even though that's what we want in the AST in almost every case.