[lld][WebAssembly] Honor command line reloc model during LTO (#164838)

This code matches what the ELF linker already does. See ae4c30a from
back in 2019.

Author: sbc100

lld/test/wasm/lto/relocation-model.ll

@@ -0,0 +1,21 @@
+;; The explicit relocation model flag.
+
+; RUN: llvm-as %s -o %t.o
+
+; RUN: wasm-ld %t.o -o %t.wasm -save-temps -r -mllvm -relocation-model=pic
+; RUN: llvm-readobj -r %t.wasm.lto.o | FileCheck %s --check-prefix=PIC
+
+; RUN: wasm-ld %t.o -o %t_static.wasm -save-temps -r -mllvm -relocation-model=static
+; RUN: llvm-readobj -r %t_static.wasm.lto.o | FileCheck %s --check-prefix=STATIC
+
+; PIC: R_WASM_GLOBAL_INDEX_LEB foo
+; STATIC: R_WASM_MEMORY_ADDR_LEB foo
+
+target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-n32:64-S128-ni:1:10:20"
+target triple = "wasm32-unknown-unknown"
+
+@foo = external global i32
+define i32 @_start() {
+  %t = load i32, i32* @foo
+  ret i32 %t
+}

lld/wasm/LTO.cpp

@@ -57,7 +57,9 @@ static lto::Config createConfig() {
   c.DebugPassManager = ctx.arg.ltoDebugPassManager;
   c.AlwaysEmitRegularLTOObj = !ctx.arg.ltoObjPath.empty();
 
-  if (ctx.arg.relocatable)
+  if (auto relocModel = getRelocModelFromCMModel())
+    c.RelocModel = *relocModel;
+  else if (ctx.arg.relocatable)
     c.RelocModel = std::nullopt;
   else if (ctx.isPic)
     c.RelocModel = Reloc::PIC_;