[ELF] - Combine LinkerScriptBase and LinkerScript<ELFT>

Patch removes templated linkerscript class.

Unfortunately that required 2 additional static methods
findSymbol() and addRegularSymbol() because code
depends on Symtab<ELFT>::X

Differential revision: https://reviews.llvm.org/D30982

llvm-svn: 298241
diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp
index 88e709b..102af08 100644
--- a/lld/ELF/SyntheticSections.cpp
+++ b/lld/ELF/SyntheticSections.cpp
@@ -2199,7 +2199,7 @@
 
 void MipsRldMapSection::writeTo(uint8_t *Buf) {
   // Apply filler from linker script.
-  uint64_t Filler = ScriptBase->getFiller(this->Name);
+  uint64_t Filler = Script->getFiller(this->Name);
   Filler = (Filler << 32) | Filler;
   memcpy(Buf, &Filler, getSize());
 }