[Debug] Print only description of value types that conform to CustomStringConvertible

[kastiglione]

This is for lldb, where the po command for Swift is implemented in _DebuggerSupport.stringForPrintObject.

Up to now, a Swift struct which conforms to CustomStringConvertible will print both its description, and also recursively print its internals.

This differs from a class, where only the description is printed.

This changes stringForPrintObject to print only the description of value types that conform to CustomStringConvertible (or debugDescription for CustomDebugStringConvertible) – unless the type conforms to CustomReflectable, in which case its children are printed.

This issue was raised for FilePath, which prints excessively verbosely, like so:

(lldb) po FilePath("/tmp")
▿ "/tmp"
  ▿ _storage : "/tmp"
    ▿ nullTerminatedStorage : 5 elements
      ▿ 0 : SystemChar
        - rawValue : 47
      ▿ 1 : SystemChar
        - rawValue : 116
      ▿ 2 : SystemChar
        - rawValue : 109
      ▿ 3 : SystemChar
        - rawValue : 112
      ▿ 4 : SystemChar
        - rawValue : 0

rdar://161919005

[kastiglione]

@swift-ci smoke test

[kastiglione]

swiftlang/llvm-project#11570

@swift-ci smoke test

[kastiglione]

swiftlang/llvm-project#11570

@swift-ci smoke test

[kastiglione]

swiftlang/llvm-project#11570

@swift-ci smoke test

[kastiglione]

swiftlang/llvm-project#11570

@swift-ci smoke test Linux

[kastiglione]

swiftlang/llvm-project#11570

@swift-ci smoke test