Devirtualize ScriptParserBase. NFC.

ScriptParserBase class is a container of collection of various methods
to parse linker script-ish text. It had a virtual method `run` to run
the parser. But we don't have to enforce its descendents to implement
that. It's up to them.

This patch removes pure virtual function `run`.

llvm-svn: 267246
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 44246f4..5fad6e1 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -317,7 +317,7 @@
 public:
   ScriptParser(StringRef S, bool B) : ScriptParserBase(S), IsUnderSysroot(B) {}
 
-  void run() override;
+  void run();
 
 private:
   void addFile(StringRef Path);