[ELF] - Check the input bitcode files for compatibility.

[grimar]

Summary

Previously BC files were not checked for the same platform etc,
That lead to confusing error "Invalid section header entry size (e_shentsize) in ELF header" when
mixing files for different architectures.

Patch fixes PR28324.

[ruiu]

LGTM. Thank you for doing this.

ELF/SymbolTable.cpp

 using namespace llvm::ELF;
 
 using namespace lld;
 using namespace lld::elf;
 
 // All input object files must be for the same architecture
 // (e.g. it does not make sense to link x86 object files with
 // MIPS object files.) This function checks for that error.
-template <class ELFT> static bool isCompatible(InputFile *FileP) {
-  auto *F = dyn_cast<ELFFileBase<ELFT>>(FileP);
-  if (!F)
+template <class ELFT> static bool isCompatible(InputFile *F) {
+  if (!isa<ELFFileBase<ELFT>>(F) && !isa<BitcodeFile>(F))
     return true;
   if (F->EKind == Config->EKind && F->EMachine == Config->EMachine)
     return true;
   StringRef A = F->getName();
   StringRef B = Config->Emulation;
   if (B.empty())
     B = Config->FirstElf->getName();
   error(A + " is incompatible with " + B);

test/ELF/lto/mix-platforms.ll

+; REQUIRES: x86
+; RUN: llvm-mc %p/Inputs/shared.s -o %t386.o -filetype=obj -triple=i386-pc-linux
+; RUN: ld.lld %t386.o -o %ti386.so -shared
+; RUN: llvm-as %s -o %tx64.o
+; RUN: not ld.lld %ti386.so %tx64.o -o %t 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: {{.*}}x64.o is incompatible with {{.*}}i386.so