[lldb][test] Compile with -parse-as-library for test main.swift sources containing @main

[kastiglione]

Update Swift.rules to automatically use the -parse-as-library compiler flag when the following conditions are true:

1. A source file is named main.swift
2. main.swift contains the @main marker

[kastiglione]

@swift-ci test

[kastiglione]

@swift-ci test

[felipepiovezan]

Out of curiosity, what is the benefit of this?

[kastiglione]

If I'm understanding the question: When using @main, you must also use -parse-as-library.

With this change, tests can use @main without having to explicitly add SWIFTFLAGS_EXTRAS := -parse-as-library to its Makefile.

Demonstration:

% swiftc main.swift
main.swift:1:1: error: 'main' attribute cannot be used in a module that contains top-level code
1 | @main struct Entry {
  | |- error: 'main' attribute cannot be used in a module that contains top-level code
  | |- note: top-level code defined in this source file
  | `- note: pass '-parse-as-library' to compiler invocation if this is intentional
2 |   static func main() {
3 |   }