[ELF] Simplify SectionBase::partition handling and make it live by default. NFC

Previously an InputSectionBase is dead (`partition==0`) by default.
SyntheticSection calls markLive and BssSection overrides that with markDead.

It is more natural to make InputSectionBase live by default and let
--gc-sections mark InputSectionBase dead.

When linking a Release build of clang:

* --no-gc-sections:, the removed `inputSections` loop decreases markLive time from 4ms to 1ms.
* --gc-sections: the extra `inputSections` loop increases markLive time from 0.181296s to 0.188526s.
  This is as of we lose the removing one `inputSections` loop optimization (4374824ccf).
  I believe the loss can be mitigated if we refactor markLive.

lld/ELF/Driver.cpp

@@ -1828,7 +1828,6 @@ static void replaceCommonSymbols() {
 
       auto *bss = make<BssSection>("COMMON", s->size, s->alignment);
       bss->file = s->file;
-      bss->markDead();
       inputSections.push_back(bss);
       s->replace(Defined{s->file, s->getName(), s->binding, s->stOther, s->type,
                          /*value=*/0, s->size, bss});

lld/ELF/InputSection.h

@@ -65,7 +65,7 @@ public:
   // The 1-indexed partition that this section is assigned to by the garbage
   // collector, or 0 if this section is dead. Normally there is only one
   // partition, so this will either be 0 or 1.
-  uint8_t partition;
+  uint8_t partition = 1;
   elf::Partition &getPartition() const;
 
   // These corresponds to the fields in Elf_Shdr.
@@ -96,8 +96,8 @@ protected:
                         uint32_t entsize, uint32_t alignment, uint32_t type,
                         uint32_t info, uint32_t link)
       : name(name), sectionKind(sectionKind), bss(false), keepUnique(false),
-        partition(0), alignment(alignment), flags(flags), entsize(entsize),
-        type(type), link(link), info(info) {}
+        alignment(alignment), flags(flags), entsize(entsize), type(type),
+        link(link), info(info) {}
 };
 
 // This corresponds to a section of an input file.

lld/ELF/MarkLive.cpp

@@ -371,9 +371,6 @@ template <class ELFT> void elf::markLive() {
   llvm::TimeTraceScope timeScope("markLive");
   // If --gc-sections is not given, retain all input sections.
   if (!config->gcSections) {
-    for (InputSectionBase *sec : inputSections)
-      sec->markLive();
-
     // If a DSO defines a symbol referenced in a regular object, it is needed.
     for (Symbol *sym : symtab->symbols())
       if (auto *s = dyn_cast<SharedSymbol>(sym))
@@ -382,6 +379,9 @@ template <class ELFT> void elf::markLive() {
     return;
   }
 
+  for (InputSectionBase *sec : inputSections)
+    sec->markDead();
+
   // Follow the graph to mark all live sections.
   for (unsigned curPart = 1; curPart <= partitions.size(); ++curPart)
     MarkLive<ELFT>(curPart).run();

lld/ELF/SyntheticSections.h

@@ -40,9 +40,7 @@ public:
   SyntheticSection(uint64_t flags, uint32_t type, uint32_t alignment,
                    StringRef name)
       : InputSection(nullptr, flags, type, alignment, {}, name,
-                     InputSectionBase::Synthetic) {
-    markLive();
-  }
+                     InputSectionBase::Synthetic) {}
 
   virtual ~SyntheticSection() = default;
   virtual void writeTo(uint8_t *buf) = 0;

lld/ELF/Writer.cpp

@@ -315,8 +315,7 @@ template <class ELFT> void elf::createSyntheticSections() {
   // If there is a SECTIONS command and a .data.rel.ro section name use name
   // .data.rel.ro.bss so that we match in the .data.rel.ro output section.
   // This makes sure our relro is contiguous.
-  bool hasDataRelRo =
-      script->hasSectionsCommand && findSection(".data.rel.ro", 0);
+  bool hasDataRelRo = script->hasSectionsCommand && findSection(".data.rel.ro");
   in.bssRelRo = std::make_unique<BssSection>(
       hasDataRelRo ? ".data.rel.ro.bss" : ".bss.rel.ro", 0, 1);
   add(*in.bssRelRo);