Instantiate Interp output section only when needed.

This change simplifies interaction between Writer and the linker script
because we can make needsInterpSection() a file-scope function.

llvm-svn: 276261
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 5c9edea..7ebd9b5 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -344,7 +344,7 @@
 
     switch (Cmd.Type) {
     case PT_INTERP:
-      if (needsInterpSection<ELFT>())
+      if (Out<ELFT>::Interp)
         Added.add(Out<ELFT>::Interp);
       break;
     case PT_DYNAMIC: