[WebAssembly] Improve lto/comdat.ll test. NFC.

[sbc100]

Summary

We were not previously testing the comdat exclusion with bitcode
object parsing because we were linking two copied of the .bc file and
the linkonce_odr linkage type was removing the duplicate _start
function during LTO.

Now we link an bitcode and non-bitcode version both of which contains
a copy of _start. And we link them in both orders, which means this
test will fail if comdat exclusion is not working correctly in the
parsing of the bitcode file.

lld/trunk/test/wasm/lto/comdat.ll

 ; Verify that comdat symbols can be defined in LTO objects.  We had a
 ; regression where the comdat handling code was causing symbol in the lto object
 ; to be ignored.
-; RUN: llvm-as %s -o %t.o
-; RUN: wasm-ld %t.o %t.o -o %t.wasm
+; RUN: llvm-as %s -o %t.bc
+; RUN: llc -filetype=obj %s -o %t.o
+; RUN: wasm-ld %t.bc %t.o -o %t.wasm
+; RUN: wasm-ld %t.o %t.bc -o %t.wasm
 
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32-unknown-unknown"
 
 $foo = comdat any
 
-define linkonce_odr void @_start() comdat($foo) {
+define void @_start() comdat($foo) {
 entry:
   ret void
 }