[LTO] Support LLVM LTO for IRGen and frontend

This commit adds -lto flag for frontend to enable LTO at LLVM level.
When -lto=llvm given, compiler emits LLVM bitcode file instead of object
file and adds index summary for LTO.
In addition for ELF format, emit llvm.dependent-libraries section to
embed auto linking information

lib/Frontend/CompilerInvocation.cpp

@@ -1474,6 +1474,18 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     }
   }
 
+  if (const Arg *A = Args.getLastArg(options::OPT_lto)) {
+    auto LLVMLTOKind =
+        llvm::StringSwitch<Optional<IRGenLLVMLTOKind>>(A->getValue())
+            .Case("llvm-thin", IRGenLLVMLTOKind::Thin)
+            .Case("llvm-full", IRGenLLVMLTOKind::Full)
+            .Default(llvm::None);
+    if (LLVMLTOKind)
+      Opts.LLVMLTOKind = LLVMLTOKind.getValue();
+    else
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                     A->getAsString(Args), A->getValue());
+  }
 
   if (const Arg *A = Args.getLastArg(options::OPT_sanitize_coverage_EQ)) {
     Opts.SanitizeCoverage =