Give a more specific diagnostic when file type collides with non-file type in same file

[RikkiGibson]

Related to #60819

roslyn/src/Compilers/CSharp/Test/Symbol/Symbols/Source/FileModifierTests.cs

Lines 1072 to 1077 in a540733

	// PROTOTYPE(ft): should this diagnostic be more specific?
	// the issue more precisely is that a definition for 'C' already exists in the current file--not that it's already in this namespace.
	comp.VerifyDiagnostics(
	// (8,12): error CS0101: The namespace '<global namespace>' already contains a definition for 'C'
	// file class C
	Diagnostic(ErrorCode.ERR_DuplicateNameInNS, "C").WithArguments("C", "<global namespace>").WithLocation(8, 12));