[ELF] Fix crash when an input is incompatible with a lazy object file

The diagnostic is concise. It is ok because the case is rare.

(cherry picked from commit bad1b7f)

lld/ELF/InputFiles.cpp

@@ -152,18 +152,17 @@ static bool isCompatible(InputFile *file) {
     return false;
   }
 
-  InputFile *existing;
+  InputFile *existing = nullptr;
   if (!objectFiles.empty())
     existing = objectFiles[0];
   else if (!sharedFiles.empty())
     existing = sharedFiles[0];
   else if (!bitcodeFiles.empty())
     existing = bitcodeFiles[0];
-  else
-    llvm_unreachable("Must have -m, OUTPUT_FORMAT or existing input file to "
-                     "determine target emulation");
-
-  error(toString(file) + " is incompatible with " + toString(existing));
+  std::string with;
+  if (existing)
+    with = " with " + toString(existing);
+  error(toString(file) + " is incompatible" + with);
   return false;
 }
 

lld/test/ELF/incompatible-ar-first.s

@@ -10,3 +10,6 @@
 // * -m was not used.
 // CHECK: .a({{.*}}a.o) is incompatible with {{.*}}b.o
 
+// RUN: not ld.lld --start-lib %ta.o --end-lib %tb.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK2
+
+// CHECK2: {{.*}}b.o is incompatible{{$}}