[ELF] Don't create an output section named `/DISCARD/` if it is assigned to the special phdr `NONE`

Fixes the remaining issue of PR41673 after D61186: with `/DISCARD/ { ... } :NONE`,
we may create an output section named `/DISCARD/`.

Note, if an input section is named `/DISCARD/`, ld.bfd discards it but
lld keeps it. It is probably not worth copying this behavior as it is unrealistic.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D62768

llvm-svn: 362356

lld/ELF/LinkerScript.cpp

@@ -824,6 +824,9 @@ void LinkerScript::assignOffsets(OutputSection *Sec) {
 }
 
 static bool isDiscardable(OutputSection &Sec) {
+  if (Sec.Name == "/DISCARD/")
+    return true;
+
   // We do not remove empty sections that are explicitly
   // assigned to any segment.
   if (!Sec.Phdrs.empty())

lld/test/ELF/linkerscript/discard-phdr.s

@@ -10,7 +10,7 @@
 # RUN:  /DISCARD/ : { *(.discard) } :NONE \
 # RUN: }" > %t.script
 # RUN: ld.lld -o %t --script %t.script %t.o
-# RUN: llvm-readelf -S -l %t | FileCheck %s
+# RUN: llvm-readelf -S -l %t | FileCheck --implicit-check-not=/DISCARD/ %s
 
 ## Check that /DISCARD/ does not interfere with the assignment of segments to
 ## sections.