[ELF] - Linkerscript: disallow discarding COMMON.

This patch restricts following construction:

/DISCARD/ : { *(COMMON) }

Previously LLD would crash.

Differential revision: https://reviews.llvm.org/D36468

llvm-svn: 310554
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 1fb787a..805d50b 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -319,8 +319,8 @@
 void LinkerScript::discard(ArrayRef<InputSectionBase *> V) {
   for (InputSectionBase *S : V) {
     S->Live = false;
-    if (S == InX::ShStrTab || S == InX::Dynamic || S == InX::DynSymTab ||
-        S == InX::DynStrTab)
+    if (S == InX::ShStrTab || S == InX::Common || S == InX::Dynamic ||
+        S == InX::DynSymTab || S == InX::DynStrTab)
       error("discarding " + S->Name + " section is not allowed");
     discard(S->DependentSections);
   }