[SIL][Frontend] Simplify debug info generation flow for SIL files

[mshockwave]

• If any of the -g<kind> flag is given -- except -gnone, debug info will be printed into every generated SIL files.
• The -gsil is deprecated in favor of -sil-based-debuginfo to avoid confusion with -g, -gnone, -gline-tables-only etc. The SILDebugInfoGenerator Pass now generates intermediate SIL file with name ".sil_dbg_.sil". Other functionalities of that Pass remain the same.

For -sil-based-debuginfo, I tried to avoid generating intermediate SIL file (i.e. *.sil_dbg_*.sil) if the input is already a SIL file -- the idea was to modify SILParser if the input is SIL. But it turned out to take non-trivial amount of changes in SILParser, which is probably not worth it for a feature that is less-frequently used. So I eventually fallback to using the existing round-trip-solution by SILDebugInfoGenerator, which always generates an intermediates SIL file, regardless of the input file type.

This PR should be synchronized with swiftlang/swift-driver#737

[mshockwave]

@swift-ci test

[adrian-prantl]

Thanks, this looks good, and is makes the options less confusion.

[mshockwave]

Alright, instead of removing -gsil, now I keep the flag, make it no-op, and print a warning message regarding the deprecation. Not sure if this is the best solution so feel free to provide some suggestions here.

[mshockwave]

@swift-ci test

[mshockwave]

I just simplified the flags passed from driver: No extra -Xllvm -sil-print-debuginfo flags will be passed from driver anymore if -g is given. Instead, if any of the debugger level is given, the frontend will populate the newly-added SILOptions::PrintDebugInfo flag, which will later be used by SILPrinter to print debug info into SIL files.

[mshockwave]

@swift-ci test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 73fd2319da0edca75742285c1eee61e57d893d6f

[mshockwave]

Fix test failure in SILOptimizer/outliner.swift by updating the CHECK directive to consider the debug location.
@swift-ci test