Implement conditional stripping of type descriptors, protocols and protocol conformances via !llvm.used.conditional

A new LLVM IR affordance that allows expressing conditions under which globals
can be removed/dropped (even when marked with @llvm.used) is being discussed at:

- <https://reviews.llvm.org/D104496>
- <https://lists.llvm.org/pipermail/llvm-dev/2021-September/152656.html>

This is a preliminary implementation that marks runtime lookup records (namely
protocol records, type descriptors records and protocol conformance records)
with the !llvm.used.conditional descriptors. That allows link-time / LTO-time
removal of these records (by GlobalDCE) based on whether they're actually used
within the linkage unit. Effectively, this allows libraries that have a limited
and known set of clients, to be optimized against the client at LTO time, and
significantly reduce the code size of that library.

Parts of the implementation:
- New -conditional-runtime-records frontend flag to enable using !llvm.used.conditional
- IRGen code that emits these records can now emit these either as a single contiguous
  array (asContiguousArray = true, the old way), which is used for JIT mode, or
  as indivial globals (asContiguousArray = false), which is necessary for the
  !llvm.used.conditional stripping to work.
- When records are emitted as individual globals, they have new names of
  "\01l_protocol_" + mangled name of the protocol descriptor, and similarly for
  other records.
- Fixed existing tests to account for individual records instead of a single array
- Added an IR level test, and an end-to-end execution test to demonstrate that
  the !llvm.used.conditional-based stripping actually works.

Author: kubamracek

include/swift/AST/IRGenOptions.h

@@ -361,6 +361,8 @@ class IRGenOptions {
 
   unsigned WitnessMethodElimination : 1;
 
+  unsigned ConditionalRuntimeRecords : 1;
+
   unsigned InternalizeAtLink : 1;
 
   /// The number of threads for multi-threaded code generation.
@@ -422,7 +424,8 @@ class IRGenOptions {
         DisableRoundTripDebugTypes(false), DisableDebuggerShadowCopies(false),
         DisableConcreteTypeMetadataMangledNameAccessors(false),
         EnableGlobalISel(false), VirtualFunctionElimination(false),
-        WitnessMethodElimination(false), InternalizeAtLink(false),
+        WitnessMethodElimination(false), ConditionalRuntimeRecords(false),
+        InternalizeAtLink(false),
         CmdArgs(),
         SanitizeCoverage(llvm::SanitizerCoverageOptions()),
         TypeInfoFilter(TypeInfoDumpFilter::All) {}

include/swift/Option/FrontendOptions.td

@@ -507,6 +507,10 @@ def internalize_at_link : Flag<["-"], "internalize-at-link">,
   " all client code of public types is part of the same link unit, or that"
   " external symbols are explicitly requested via -exported_symbols_list)">;
 
+def conditional_runtime_records : Flag<["-"], "conditional-runtime-records">,
+  Flags<[FrontendOption, NoInteractiveOption, HelpHidden]>,
+  HelpText<"Allow removal of runtime metadata records (public types, protocol conformances) based on whether they're used or unused">;
+
 def disable_previous_implementation_calls_in_dynamic_replacements :
   Flag<["-"], "disable-previous-implementation-calls-in-dynamic-replacements">,
   Flags<[FrontendOption, NoInteractiveOption, HelpHidden]>,

lib/Frontend/CompilerInvocation.cpp

@@ -1934,6 +1934,10 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.WitnessMethodElimination = true;
   }
 
+  if (Args.hasArg(OPT_conditional_runtime_records)) {
+    Opts.ConditionalRuntimeRecords = true;
+  }
+
   if (Args.hasArg(OPT_internalize_at_link)) {
     Opts.InternalizeAtLink = true;
   }