Provide FSharpParameter with its correct list of FSharpAttributes

[baronfel]

Is your feature request related to a problem? Please describe.

I was implementing nested-language support in FSAC by checking if parameters of methods are decorated with System.Diagnostics.CodeAnalysis.StringSyntaxAttribute, and I was doing this by

• getting the ident for the method being called
• getting the symbol use for that ident
• getting the member, function, or value for that symbol use
• investigating the parameter list for that MFV
• checking the attributes on the parameter in the correct position

However, FSharpParameter seems to always have an empty .Attributes member, as shown in

fsharp/src/Compiler/Symbols/Symbols.fs

Lines 2077 to 2079 in 26b8772

	// INCOMPLETENESS: Attribs is empty here, so we can't look at attributes for
	// either .NET or F# parameters
	let argInfo: ArgReprInfo = { Name=nmOpt; Attribs=[]; OtherRange=None }

and a few lines below.

Describe the solution you'd like

I'd like .Attributes to have the list of attributes for that parameter, or have a method that could be called to lazily-calculate the list if that would increase memory usage too much.

Describe alternatives you've considered

Some horrific hacking to use reflection-only loads to try and load and investigate the matching parameter.

Additional context

Here's the tracking issue in Ionide for adding this nested-language feature: ionide/ionide-vscode-fsharp#1912

[baronfel]

Closing as duplicate of #13786