llvm-lto: default Relocation Model should be selected by the TargetMachine.

Right now, the createTargetMachine function in LTOBackend.cpp (used by llvm-lto, and other components) selects the default Relocation Model when none is specified in the module.
Other components (such as opt and llc) that construct a TargetMachine delegate the decision on the default value to the polymorphic TargetMachine's constructor.

This commit aligns llvm-lto with other components.

Reviewed By: daltenty, fhahn

Differential Revision: https://reviews.llvm.org/D97507

Author: Wael Yehia

llvm/lib/LTO/LTOBackend.cpp

@@ -184,10 +184,10 @@ createTargetMachine(const Config &Conf, const Target *TheTarget, Module &M) {
   for (const std::string &A : Conf.MAttrs)
     Features.AddFeature(A);
 
-  Reloc::Model RelocModel;
+  Optional<Reloc::Model> RelocModel = None;
   if (Conf.RelocModel)
     RelocModel = *Conf.RelocModel;
-  else
+  else if (M.getModuleFlag("PIC Level"))
     RelocModel =
         M.getPICLevel() == PICLevel::NotPIC ? Reloc::Static : Reloc::PIC_;
 

test/tools/llvm-lto/aix.ll

@@ -0,0 +1,12 @@
+; REQUIRES: powerpc-registered-target
+; RUN: llvm-as < %s > %t1
+; RUN: llvm-lto %t1 | FileCheck %s
+
+target triple = "powerpc-ibm-aix"
+
+define i32 @main() {
+entry:
+  ret i32 42
+}
+; CHECK: Wrote native object file
+