[IRGen] Set TargetOpts.UseInitArray in getIRTargetOptions().

IRGen sets up the `llvm::TargetOptions` by itself, rather than copying
them from the Clang instance (it has to do this, at present, because
Clang doesn't provide a way to get it to initialise one).

Unfortunately, it didn't set `UseInitArray`, which when linking with
GNU `ld` or `gold` is *fine* because those linkers automatically
convert `.ctors` and `.dtors` sections into `.init_array` and
`.fini_array` respectively.  *However*, `lld` does *not* do this
conversion, so when using `lld`, if the compiler generates a
constructor or destructor function, it won't be called(!)

The fix is to set `UseInitArray` properly; I chose to copy the setting
from Clang, just in case Clang knows of a reason why it shouldn't be
`true`.

rdar://123504095

Author: al45tair

lib/IRGen/IRGen.cpp

@@ -138,6 +138,9 @@ swift::getIRTargetOptions(const IRGenOptions &Opts, ASTContext &Ctx) {
 
   auto *Clang = static_cast<ClangImporter *>(Ctx.getClangModuleLoader());
 
+  // Set UseInitArray appropriately.
+  TargetOpts.UseInitArray = Clang->getCodeGenOpts().UseInitArray;
+
   // WebAssembly doesn't support atomics yet, see
   // https://github.com/apple/swift/issues/54533 for more details.
   if (Clang->getTargetInfo().getTriple().isOSBinFormatWasm())