[AsmParser] Remove unused forward declarations. NFC.

Author: RKSimon

llvm/lib/AsmParser/LLLexer.h

@@ -20,7 +20,6 @@
 #include <string>
 
 namespace llvm {
-  class MemoryBuffer;
   class Type;
   class SMDiagnostic;
   class LLVMContext;

llvm/lib/AsmParser/LLParser.h

@@ -27,7 +27,6 @@
 
 namespace llvm {
   class Module;
-  class OpaqueType;
   class Function;
   class Value;
   class BasicBlock;
@@ -38,7 +37,6 @@ namespace llvm {
   class MDString;
   class MDNode;
   struct SlotMapping;
-  class StructType;
 
   /// ValID - Represents a reference of a definition of some sort with no type.
   /// There are several cases where we have to parse the value but where the