[ORC] Support adding LC_LOAD_WEAK_DYLIB commands to MachO JITDylib headers.

MachOPlatform synthesizes Mach headers for JITDylibs (see ef314d3). This
commit adds support for adding LC_LOAD_WEAK_DYLIB commands to these synthesized
headers (LC_LOAD_DYLIB was already supported previously).

Author: lhames

llvm/include/llvm/ExecutionEngine/Orc/MachOBuilder.h

@@ -116,6 +116,12 @@ struct MachOBuilderLoadCommand<MachO::LC_LOAD_DYLIB>
   using MachOBuilderDylibLoadCommand::MachOBuilderDylibLoadCommand;
 };
 
+template <>
+struct MachOBuilderLoadCommand<MachO::LC_LOAD_WEAK_DYLIB>
+    : public MachOBuilderDylibLoadCommand<MachO::LC_LOAD_WEAK_DYLIB> {
+  using MachOBuilderDylibLoadCommand::MachOBuilderDylibLoadCommand;
+};
+
 template <>
 struct MachOBuilderLoadCommand<MachO::LC_RPATH>
     : public MachOBuilderLoadCommandImplBase<MachO::LC_RPATH> {

llvm/include/llvm/ExecutionEngine/Orc/MachOPlatform.h

@@ -58,6 +58,13 @@ class MachOPlatform : public Platform {
       uint32_t CompatibilityVersion;
     };
 
+    struct LoadDylibCmd {
+      enum class LoadKind { Default, Weak };
+
+      Dylib D;
+      LoadKind K;
+    };
+
     struct BuildVersionOpts {
 
       // Derive platform from triple if possible.
@@ -74,7 +81,7 @@ class MachOPlatform : public Platform {
     std::optional<Dylib> IDDylib;
 
     /// List of LC_LOAD_DYLIBs.
-    std::vector<Dylib> LoadDylibs;
+    std::vector<LoadDylibCmd> LoadDylibs;
     /// List of LC_RPATHs.
     std::vector<std::string> RPaths;
     /// List of LC_BUILD_VERSIONs.

llvm/lib/ExecutionEngine/Orc/MachOPlatform.cpp

@@ -1763,9 +1763,22 @@ jitlink::Block &createHeaderBlock(MachOPlatform &MOP,
   for (auto &BV : Opts.BuildVersions)
     B.template addLoadCommand<MachO::LC_BUILD_VERSION>(
         BV.Platform, BV.MinOS, BV.SDK, static_cast<uint32_t>(0));
-  for (auto &D : Opts.LoadDylibs)
-    B.template addLoadCommand<MachO::LC_LOAD_DYLIB>(
-        D.Name, D.Timestamp, D.CurrentVersion, D.CompatibilityVersion);
+
+  using LoadKind = MachOPlatform::HeaderOptions::LoadDylibCmd::LoadKind;
+  for (auto &LD : Opts.LoadDylibs) {
+    switch (LD.K) {
+    case LoadKind::Default:
+      B.template addLoadCommand<MachO::LC_LOAD_DYLIB>(
+          LD.D.Name, LD.D.Timestamp, LD.D.CurrentVersion,
+          LD.D.CompatibilityVersion);
+      break;
+    case LoadKind::Weak:
+      B.template addLoadCommand<MachO::LC_LOAD_WEAK_DYLIB>(
+          LD.D.Name, LD.D.Timestamp, LD.D.CurrentVersion,
+          LD.D.CompatibilityVersion);
+      break;
+    }
+  }
   for (auto &P : Opts.RPaths)
     B.template addLoadCommand<MachO::LC_RPATH>(P);