[SR-7904] Add source filenames in the output of -dump-ast

[fedetrim]

Text by Vinicius Vendramini in SR-7904:

The Swift compiler's -dump-ast functionality prints an output that doesn't include the file names, which can make it impossible to distinguish between different files.
For instance, when compiling two files a.swift and b.swift, both containing the code "private func foo() { }", with the command "$ swiftc b.swift a.swift -dump-ast", we get this output:

(source_file
  (func_decl "foo()" interface type='() -> ()' access=private
    (parameter_list)
    (brace_stmt)))
(source_file
  (func_decl "foo()" interface type='() -> ()' access=private
    (parameter_list)
    (brace_stmt)))

It would be very useful if this information included the file name (or, even better, the file path). Something like this:

(source_file "/path/to/a.swift"
  (func_decl "foo()" interface type='() -> ()' access=private
    (parameter_list)
    (brace_stmt)))
(source_file "/path/to/b.swift"
  (func_decl "foo()" interface type='() -> ()' access=private
    (parameter_list)
    (brace_stmt)))

Resolves SR-7904.

[beccadax]

This looks like a great start! It looks like it doesn't quote the path, though; I think that would be better, since other places where we put an arbitrary string into the AST usually quote them.

[fedetrim]

@brentdax you are right! Fixed that, and also removed my FileColor in favour of the already existing LocationColor. Let me know if that's OK or another color is needed/wanted here. Thanks!

[jckarter]

@swift-ci Please smoke test

[fedetrim]

@jckarter @brentdax hey guys! I see the smoke tests passed successfully, is there anything else I can do to improve this PR?

[jckarter]

Looks good, thanks for reminding me to commit it!