[ELF] - Allow producing -r output if only empty archive is given.

This is used by linux kernel build system.

(https://www.kernel.org/doc/Documentation/kbuild/makefiles.txt "3.2 Built-in object goals")

It has for example next configuration for linking built-in.o files:
drivers-y	:= $(patsubst %/, %/built-in.o, $(drivers-y))
drivers-$(CONFIG_PCI)		+= arch/ia64/pci/
...
drivers-$(CONFIG_OPROFILE)	+= arch/ia64/oprofile/

Im most simple case all CONFIG_* options are off. That means linker is called with empty input archive, 
emulation option and no inputs and expected to generate some relocatable output. 
ld.bfd is able to do that, we dont.

Patch allows to support this case.

Differential revision: https://reviews.llvm.org/D33937

llvm-svn: 305069

lld/ELF/Driver.cpp

@@ -185,7 +185,7 @@ void LinkerDriver::addFile(StringRef Path, bool WithLOption) {
     // is attempting LTO and using a default ar command that doesn't
     // understand the LLVM bitcode file. It is a pretty common error, so
     // we'll handle it as if it had a symbol table.
-    if (!File->hasSymbolTable()) {
+    if (!File->isEmpty() && !File->hasSymbolTable()) {
       for (const auto &P : getArchiveMembers(MBRef))
         Files.push_back(make<LazyObjectFile>(P.first, Path, P.second));
       return;

lld/test/ELF/relocatable-empty-archive.s

@@ -0,0 +1,10 @@
+# REQUIRES: x86
+# RUN: rm %t.a
+# RUN: llvm-ar rc %t.a
+# RUN: ld.lld -m elf_x86_64 %t.a -o %t -r
+# RUN: llvm-readobj -file-headers %t | FileCheck %s
+
+# CHECK: Format: ELF64-x86-64
+# CHECK: Arch: x86_64
+# CHECK: AddressSize: 64bit
+# CHECK: Type: Relocatable