Add introspection macros support for dot syntax

[Liozou]

Fixes #28615, fixes #14637, closes #20620. Actually the last one was already fixed on master it seems, so I only added a test.

Broadcasting

This PR enables using the code_ macros with dot expressions, like so:

julia> @code_warntype max.(1 .+ 3, 5 - 7)
Variables
  #self#::Core.Compiler.Const(var"##dotfunction#282#116"(), false)
  x1::Int64
  x2::Int64
  x3::Int64

Body::Int64
1 ─ %1 = Base.broadcasted(Main.:+, x1, x2)::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{0},Nothing,typeof(+),Tuple{Int64,Int64}}
│   %2 = Base.broadcasted(Main.max, %1, x3)::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{0},Nothing,typeof(max),Tuple{Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{0},Nothing,typeof(+),Tuple{Int64,Int64}},Int64}}
│   %3 = Base.materialize(%2)::Int64
└──      return %3

This works by wrapping the dot expression in a function and applying the introspection to the created function. In the example above, the newly created function is

dotfunction(x1, x2, x3) = max.(x1 .+ x2, x3)

Note that the 5 - 7 argument has been transformed into a single argument for the new function, since it is not broadcasted upon.

Other introspection macros such as @which or @edit now error with a message pointing the user towards Meta.@lower:

julia> @which abs.([1,2])
ERROR: dot expressions are not lowered to a single function call, so @which cannot analyze them. You may want to use Meta.@lower to identify which function call to target.
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] top-level scope at none:1

I also added a small message mentioning Meta.@lower in the general error message for the introspection macros because I think it deserves to be there, but let me know what you think of it.

getproperty

The dot used for broadcasting and the dot used for getproperty have different context so there is no confusion when handling them. When both are present, the outermost dominates (I don't know how to phrase it, but I think it just follows intuition):

julia> @code_typed optimize=false Ref.(["foo", 4])[1].x
CodeInfo(
1 ─ %1 = Base.getfield(x, f)::String
└──      return %1
) => String

Finally, @which can now handle fully qualified symbols (#14637):

julia> @which Main.Base.nothing
Core

[MasonProtter]

This should probably have a NEWS.md entry, right?

[Liozou]

Well it's arguably a bugfix. But if people are in favor I can PR a NEWS entry joined with #35283, it would make sense.

[MasonProtter]

I see it as a feature enhancement.