[LLVMgold.so] -plugin-opt=save-temps: save combined module to .lto.o instead of .o

This matches LLD and fixes https://sourceware.org/bugzilla/show_bug.cgi?id=26262#c1

.o is a bad choice for save-temps output because it is easy to override the bitcode file (*.o)

```
 # Use bfd for the example, -fuse-ld=gold is similar.
clang -flto -c a.c  # generate bitcode file a.o
clang -fuse-ld=bfd -flto a.o -o a -Wl,-plugin-opt=save-temps  # override a.o

 # The user repeats the command but get surprised, because a.o is now a combined module.
clang -fuse-ld=bfd -flto a.o -o a -Wl,-plugin-opt=save-temps
```

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D84132

Author: MaskRay

llvm/test/tools/gold/X86/parallel.ll

@@ -1,10 +1,10 @@
 ; RUN: llvm-as -o %t.bc %s
-; RUN: rm -f %t.0.5.precodegen.bc %t.1.5.precodegen.bc %t.o %t.o1
+; RUN: rm -f %t.0.5.precodegen.bc %t.1.5.precodegen.bc %t.lto.o %t.lto.o1
 ; RUN: env LD_PRELOAD=%llvmshlibdir/LLVMgold%shlibext %gold -plugin %llvmshlibdir/LLVMgold%shlibext -u foo -u bar -plugin-opt lto-partitions=2 -plugin-opt save-temps -m elf_x86_64 -o %t %t.bc
 ; RUN: llvm-dis %t.0.5.precodegen.bc -o - | FileCheck --check-prefix=CHECK-BC0 %s
 ; RUN: llvm-dis %t.1.5.precodegen.bc -o - | FileCheck --check-prefix=CHECK-BC1 %s
-; RUN: llvm-nm %t.o | FileCheck --check-prefix=CHECK0 %s
-; RUN: llvm-nm %t.o1 | FileCheck --check-prefix=CHECK1 %s
+; RUN: llvm-nm %t.lto.o | FileCheck --check-prefix=CHECK0 %s
+; RUN: llvm-nm %t.lto.o1 | FileCheck --check-prefix=CHECK1 %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"

llvm/test/tools/gold/X86/relocation-model-pic.ll

@@ -10,44 +10,44 @@
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:    --shared \
 ; RUN:    --plugin-opt=save-temps %t.o -o %t-out
-; RUN: llvm-readobj -r %t-out.o | FileCheck %s --check-prefix=PIC
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=PIC
 
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:    --export-dynamic --noinhibit-exec -pie \
 ; RUN:    --plugin-opt=save-temps %t.o -o %t-out
-; RUN: llvm-readobj -r %t-out.o | FileCheck %s --check-prefix=PIC
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=PIC
 
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:    --export-dynamic --noinhibit-exec \
 ; RUN:    --plugin-opt=save-temps %t.o -o %t-out
-; RUN: llvm-readobj -r %t-out.o | FileCheck %s --check-prefix=STATIC
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=STATIC
 
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:    -r \
 ; RUN:    --plugin-opt=save-temps %t.o -o %t-out
-; RUN: llvm-readobj -r %t-out.o | FileCheck %s --check-prefix=STATIC
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=STATIC
 
 ;; PIC source.
 
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:    --shared \
 ; RUN:    --plugin-opt=save-temps %t.pic.o -o %t-out
-; RUN: llvm-readobj -r %t-out.o | FileCheck %s --check-prefix=PIC
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=PIC
 
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:    --export-dynamic --noinhibit-exec -pie \
 ; RUN:    --plugin-opt=save-temps %t.pic.o -o %t-out
-; RUN: llvm-readobj -r %t-out.o | FileCheck %s --check-prefix=PIC
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=PIC
 
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:    --export-dynamic --noinhibit-exec \
 ; RUN:    --plugin-opt=save-temps %t.pic.o -o %t-out
-; RUN: llvm-readobj -r %t-out.o | FileCheck %s --check-prefix=STATIC
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=STATIC
 
 ; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold%shlibext \
 ; RUN:    -r \
 ; RUN:    --plugin-opt=save-temps %t.pic.o -o %t-out
-; RUN: llvm-readobj -r %t-out.o | FileCheck %s --check-prefix=PIC
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=PIC
 
 
 ; PIC: R_X86_64_GOTPCREL foo

llvm/test/tools/gold/X86/thinlto.ll

@@ -42,7 +42,7 @@
 ; RUN: llvm-bcanalyzer -dump %t4.index.bc | FileCheck %s --check-prefix=COMBINED
 ; RUN: llvm-nm %t4 | FileCheck %s --check-prefix=NM
 ; Ensure ld does not emit empty combined module in default.
-; RUN: ls %t4.o* | count 2
+; RUN: ls %t4.lto.o* | count 2
 
 ; Check with --no-map-whole-files
 ; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext \

llvm/tools/gold/gold-plugin.cpp

@@ -1050,7 +1050,7 @@ static std::vector<std::pair<SmallString<128>, bool>> runLTO() {
   if (!options::obj_path.empty())
     Filename = options::obj_path;
   else if (options::TheOutputType == options::OT_SAVE_TEMPS)
-    Filename = output_name + ".o";
+    Filename = output_name + ".lto.o";
   else if (options::TheOutputType == options::OT_ASM_ONLY)
     Filename = output_name;
   bool SaveTemps = !Filename.empty();