[MC] Fix llvm_unreachable when a STB_GNU_UNIQUE symbol needs a relocation

STB_GNU_UNIQUE should be treated in a way similar to STB_GLOBAL.
This fixes an "Invalid Binding" failure in an LLVM_ENABLE_ASSERTIONS=on build
for source files like glibc elf/tst-unique1mod1.c .

This bug has been benign so far because (a) Clang does not produce
%gnu_unique_object by itself (b) a non-assertion build likely picks the
STB_GLOBAL code path anyway.

(cherry picked from commit 6bdad85)

Author: MaskRay

llvm/lib/MC/ELFObjectWriter.cpp

@@ -1336,6 +1336,7 @@ bool ELFObjectWriter::shouldRelocateWithSymbol(const MCAssembler &Asm,
     // can update it.
     return true;
   case ELF::STB_GLOBAL:
+  case ELF::STB_GNU_UNIQUE:
     // Global ELF symbols can be preempted by the dynamic linker. The relocation
     // has to point to the symbol for a reason analogous to the STB_WEAK case.
     return true;

llvm/test/MC/ELF/gnu-unique.s

@@ -1,12 +1,21 @@
 # RUN: llvm-mc -triple=x86_64 %s | FileCheck %s --check-prefix=ASM
-# RUN: llvm-mc -filetype=obj -triple=x86_64 %s | llvm-readelf -h -s - | FileCheck %s --check-prefix=OBJ
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t
+# RUN: llvm-readelf -h -s %t | FileCheck %s --check-prefix=OBJ
+# RUN: llvm-objdump -d -r --no-show-raw-insn %t | FileCheck %s --check-prefix=DISASM
 
 # ASM: .type unique,@gnu_unique_object
 
 # OBJ: OS/ABI: UNIX - GNU
 # OBJ: Type   Bind   Vis     Ndx Name
 # OBJ: OBJECT UNIQUE DEFAULT [[#]] unique
 
+# DISASM-LABEL: <.text>:
+# DISASM-NEXT:    movl $1, 0
+## unique has a non-local binding. Reference unique instead of .data
+# DISASM-NEXT:      R_X86_64_32S unique
+
+  movl $1, unique
+
 .data
 .globl unique
 .type unique, @gnu_unique_object