[ELF] Fix INCLUDE cycle detection

Fix #93947: the cycle detection mechanism added by
https://reviews.llvm.org/D37524 also disallowed including a file twice,
which is an unnecessary limitation.

Now that we have an include stack #100493, supporting multiple inclusion
is trivial. Note: a filename can be referenced with many different
paths, e.g. a.lds, ./a.lds, ././a.lds. We don't attempt to detect the
cycle in the earliest point.

lld/ELF/ScriptParser.cpp

@@ -24,7 +24,6 @@
 #include "lld/Common/CommonLinkerContext.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
-#include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/Support/Casting.h"
@@ -139,9 +138,6 @@ private:
   // True if a script being read is in the --sysroot directory.
   bool isUnderSysroot = false;
 
-  // A set to detect an INCLUDE() cycle.
-  StringSet<> seen;
-
   // If we are currently parsing a PROVIDE|PROVIDE_HIDDEN command,
   // then this member is set to the PROVIDE symbol name.
   std::optional<llvm::StringRef> activeProvideSym;
@@ -406,7 +402,7 @@ void ScriptParser::readGroup() {
 
 void ScriptParser::readInclude() {
   StringRef name = readName();
-  if (!seen.insert(name).second) {
+  if (!activeFilenames.insert(name).second) {
     setError("there is a cycle in linker script INCLUDEs");
     return;
   }