Make the behavior of the -v option more closer to GNU linkers.

[ruiu]

Summary

Previously, lld exited with an error status if the only option given to
the command is -v. GNU linkers gracefully exit in that case. This patch
makes lld behave like GNU.

Note that even with this patch, lld's -v and --version options behave
slightly differently than GNU linkers' counterparts. For example,
if you run ld.bfd -v -v, the version string is printed out twice.
But that is an edge case that I don't think we need to take care of.

Fixes https://bugs.llvm.org/show_bug.cgi?id=31582

lld/ELF/Driver.cpp

void LinkerDriver::main(ArrayRef<const char *> ArgsArr, bool CanExitEarly) {
   // it is not easy to improve it to recognize LLD as a GNU compatible
   // linker in a timely manner. Even if we can make it, there are still a
   // lot of "configure" scripts out there that are generated by old version
   // of Libtool. We cannot convince every software developer to migrate to
   // the latest version and re-generate scripts. So we have this hack.
   if (Args.hasArg(OPT_v) || Args.hasArg(OPT_version))
     message(getLLDVersion() + " (compatible with GNU linkers)");
 
-  // ld.bfd always exits after printing out the version string.
-  // ld.gold proceeds if a given option is -v. Because gold's behavior
-  // is more permissive than ld.bfd, we chose what gold does here.
+  // The behavior of -v or --version is a bit strange, but this is
+  // needed for compatibility with GNU linkers.
+  if (Args.hasArg(OPT_v) && !Args.hasArg(OPT_INPUT))
+    return;
   if (Args.hasArg(OPT_version))
     return;
 
   Config->ExitEarly = CanExitEarly && !Args.hasArg(OPT_full_shutdown);
   errorHandler().ExitEarly = Config->ExitEarly;
 
   if (const char *Path = getReproduceOption(Args)) {
     // Note that --reproduce is a debug option so you can ignore it

lld/test/ELF/driver.test

 # RUN: not ld.lld %t -o /no/such/file 2>&1 | FileCheck -check-prefix=MISSING %s
 # MISSING: cannot open output file /no/such/file
 
 # RUN: ld.lld --help 2>&1 | FileCheck -check-prefix=HELP %s
 # HELP: USAGE:
 # HELP: : supported targets:{{.*}} elf
 
 # RUN: ld.lld --version 2>&1 | FileCheck -check-prefix=VERSION %s
+# RUN: ld.lld -v 2>&1 | FileCheck -check-prefix=VERSION %s
+# RUN: not ld.lld -v xyz 2>&1 | FileCheck -check-prefix=VERSION %s
 # VERSION: LLD {{.*}} (compatible with GNU linkers)
 
-# RUN: not ld.lld -v 2>&1 | FileCheck -check-prefix=VERSION %s
-
 ## Attempt to link DSO with -r
 # RUN: ld.lld -shared %t -o %t.so
 # RUN: not ld.lld -r %t.so %t -o %tfail 2>&1 | FileCheck -check-prefix=ERR %s
 # ERR: attempted static link of dynamic object
 
 ## Attempt to use -r and -shared together
 # RUN: not ld.lld -r -shared %t -o %tfail 2>&1 | FileCheck -check-prefix=ERR2 %s
 # ERR2: -r and -shared may not be used together