[Driver] -c with LTO should still emit bitcode (swiftlang#35026)

Author: kubamracek

lib/Driver/Driver.cpp

@@ -1501,6 +1501,8 @@ void Driver::buildOutputInfo(const ToolChain &TC, const DerivedArgList &Args,
 
     case options::OPT_emit_object:
       OI.CompilerOutputType = file_types::TY_Object;
+      if (OI.LTOVariant != OutputInfo::LTOKind::None)
+        OI.CompilerOutputType = file_types::TY_LLVM_BC;
       break;
 
     case options::OPT_emit_assembly:

test/Driver/lto-output-mode-clash.swift

@@ -0,0 +1,13 @@
+// REQUIRES: lld_lto
+
+// RUN: %swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -lto=llvm-full -static -emit-library -target x86_64-apple-macosx10.9 | %FileCheck %s
+// RUN: %swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -lto=llvm-full -emit-library -target x86_64-apple-macosx10.9 | %FileCheck %s
+// RUN: %swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -lto=llvm-full -c -target x86_64-apple-macosx10.9 | %FileCheck %s
+// RUN: %swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -c -lto=llvm-full -target x86_64-apple-macosx10.9 | %FileCheck %s
+// RUN: %swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -c -lto=llvm-full -emit-bc -target x86_64-apple-macosx10.9 | %FileCheck %s
+// RUN: %swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -emit-bc -c -lto=llvm-full -target x86_64-apple-macosx10.9 | %FileCheck %s
+
+// CHECK: swift
+// CHECK-DAG: -emit-bc
+// CHECK-DAG: -lto=llvm-full
+// CHECK-DAG: -o [[BITCODEFILE:.*\.bc]]