[ELF] -o -: suppress output if disableOutput

So that LLD_IN_TEST=2 ld.lld -o - a.o only writes the output once.

Author: MaskRay

lld/ELF/Writer.cpp

@@ -381,9 +381,11 @@ template <class ELFT> void Writer<ELFT>::run() {
     if (errCount(ctx))
       return;
 
-    if (auto e = buffer->commit())
-      Err(ctx) << "failed to write output '" << buffer->getPath()
-               << "': " << std::move(e);
+    if (!ctx.e.disableOutput) {
+      if (auto e = buffer->commit())
+        Err(ctx) << "failed to write output '" << buffer->getPath()
+                 << "': " << std::move(e);
+    }
 
     if (!ctx.arg.cmseOutputLib.empty())
       writeARMCmseImportLib<ELFT>(ctx);

lld/test/ELF/stdout.s

@@ -1,7 +1,8 @@
 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
-# RUN: ld.lld %t.o -o - > %t1
+## Test that we only write to "-" once.
+# RUN: LLD_IN_TEST=2 ld.lld %t.o -o - > %t1
 # RUN: llvm-objdump -d %t1 | FileCheck %s
 
 # CHECK: nop