[ELF] Remove elf::config indirection. NFC

`config` has 1000+ uses so we try to avoid changing `config->foo`. Define a
wrapper with LLVM_LIBRARY_VISIBILITY to remove unneeded GOT and unique_ptr
indirection.

My x86-64 lld executable is 11+KiB smaller.

lld/ELF/Config.h

@@ -19,6 +19,7 @@
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/Support/CachePruning.h"
 #include "llvm/Support/CodeGen.h"
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/Compression.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/GlobPattern.h"
@@ -101,7 +102,7 @@ struct VersionDefinition {
 // Most fields are direct mapping from the command line options
 // and such fields have the same name as the corresponding options.
 // Most fields are initialized by the driver.
-struct Configuration {
+struct Config {
   uint8_t osabi = 0;
   uint32_t andFeatures = 0;
   llvm::CachePruningPolicy thinLTOCachePolicy;
@@ -368,9 +369,12 @@ struct Configuration {
 
   unsigned threadCount;
 };
+struct ConfigWrapper {
+  Config c;
+  Config *operator->() { return &c; }
+};
 
-// The only instance of Configuration struct.
-extern std::unique_ptr<Configuration> config;
+LLVM_LIBRARY_VISIBILITY extern ConfigWrapper config;
 
 struct DuplicateSymbol {
   const Symbol *sym;

lld/ELF/Driver.cpp

@@ -73,7 +73,7 @@ using namespace llvm::support;
 using namespace lld;
 using namespace lld::elf;
 
-std::unique_ptr<Configuration> elf::config;
+ConfigWrapper elf::config;
 std::unique_ptr<Ctx> elf::ctx;
 std::unique_ptr<LinkerDriver> elf::driver;
 
@@ -112,7 +112,7 @@ bool elf::link(ArrayRef<const char *> args, llvm::raw_ostream &stdoutOS,
   ctx->e.errorLimitExceededMsg = "too many errors emitted, stopping now (use "
                                  "--error-limit=0 to see all errors)";
 
-  config = std::make_unique<Configuration>();
+  config = ConfigWrapper();
   elf::ctx = std::make_unique<Ctx>();
   driver = std::make_unique<LinkerDriver>();
   script = std::make_unique<LinkerScript>();

lld/ELF/Relocations.cpp

@@ -444,7 +444,6 @@ public:
 private:
   InputSectionBase *sec;
   OffsetGetter getter;
-  const Configuration *const config = elf::config.get();
   const TargetInfo &target = *elf::target;
 
   // End of relocations, used by Mips/PPC64.