Bypass `SyntaxNode` in JuliaLowering; fix `kw` bug (#60162)
The `kw` form isn't produced in `SyntaxNode`/`SyntaxTree` like it is in Expr,
which simplifies the parser, but causes problems once we start start caring
about the semantics of these trees. Parsing `"f(a=1)"` will always produce an
`Expr(:call, :f, Expr(:kw, :a, 1))`, but the equivalent SyntaxNode
representation `(call f (= a 1))` makes `Expr(:call, :f, Expr(:(=), :a, 1))`
unrepresentable, even though that's valid syntax a macro could produce.
To fix this, we need to convert `=` to `kw` within certain forms before
macro expansion.
