[ELF] Fix spurious GOT/PLT assertion failure when .dynsym is discarded

Linux kernel arch/arm64/kernel/vmlinux.lds.S discards .dynsym . D123985 triggers
a spurious assertion failure. Detect the case with
`!mainPart->dynSymTab->getParent()`.

Author: MaskRay

lld/ELF/SyntheticSections.cpp

@@ -1593,7 +1593,8 @@ uint32_t DynamicReloc::getSymIndex(SymbolTableBaseSection *symTab) const {
     return 0;
 
   size_t index = symTab->getSymbolIndex(sym);
-  assert((index != 0 || type != target->gotRel && type != target->pltRel) &&
+  assert((index != 0 || type != target->gotRel && type != target->pltRel ||
+          !mainPart->dynSymTab->getParent()) &&
          "GOT or PLT relocation must refer to symbol in dynamic symbol table");
   return index;
 }

lld/test/ELF/linkerscript/discard-section-dynsym.s

@@ -0,0 +1,24 @@
+# REQUIRES: aarch64
+
+## We allow discarding .dynsym, check we don't crash.
+# RUN: llvm-mc -filetype=obj -triple=aarch64 %s -o %t.o
+
+# RUN: echo 'SECTIONS { /DISCARD/ : { *(.dynsym) } }' > %t.lds
+# RUN: ld.lld -shared -T %t.lds %t.o -o %t.so
+# RUN: llvm-readelf -r %t.so | FileCheck %s
+
+# RUN: echo 'SECTIONS { /DISCARD/ : { *(.dynsym .dynstr) } }' > %t.lds
+# RUN: ld.lld -shared -T %t.lds %t.o -o %t.so
+# RUN: llvm-readelf -r %t.so | FileCheck %s
+
+# CHECK:      contains 2 entries:
+# CHECK:      R_AARCH64_RELATIVE  [[#]]
+# CHECK-NEXT: R_AARCH64_GLOB_DAT  0{{$}}
+
+  adrp x9, :got:var
+  ldr  x9, [x9, :got_lo12:var]
+
+.data
+.align 8
+foo:
+.quad foo

lld/test/ELF/linkerscript/discard-section-err.s

@@ -11,14 +11,6 @@
 # RUN: echo "SECTIONS { /DISCARD/ : { *(.dynamic) } }" > %t.script
 # RUN: ld.lld -pie -o %t --script %t.script %t.o
 
-## We allow discarding .dynsym, check we don't crash.
-# RUN: echo "SECTIONS { /DISCARD/ : { *(.dynsym) } }" > %t.script
-# RUN: ld.lld -pie -o %t --script %t.script %t.o
-
-## We allow discarding .dynstr, check we don't crash.
-# RUN: echo "SECTIONS { /DISCARD/ : { *(.dynstr) } }" > %t.script
-# RUN: ld.lld -pie -o %t --script %t.script %t.o
-
 # RUN: echo "SECTIONS { /DISCARD/ : { *(.rela.dyn) } }" > %t.script
 # RUN: ld.lld -pie -o %t %t.o
 # RUN: llvm-readobj -S %t | FileCheck --check-prefix=RELADYN %s