Remove unused class declarations

[jankratochvil]

Summary

Simplification by removing excessive DWARFCompileUnit references for D40466 .

source/Plugins/SymbolFile/DWARF/DWARFAbbreviationDeclaration.h

 //===-- DWARFAbbreviationDeclaration.h --------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef liblldb_DWARFAbbreviationDeclaration_h_
 #define liblldb_DWARFAbbreviationDeclaration_h_
 
 #include "DWARFAttribute.h"
 #include "SymbolFileDWARF.h"
 
-class DWARFCompileUnit;
-
 class DWARFAbbreviationDeclaration {
 public:
   enum { InvalidCode = 0 };
   DWARFAbbreviationDeclaration();
 
   // For hand crafting an abbreviation declaration
   DWARFAbbreviationDeclaration(dw_tag_t tag, uint8_t has_children);
   void AddAttribute(const DWARFAttribute &attr) {

source/Plugins/SymbolFile/DWARF/HashedNameToDIE.h

 #include "lldb/Core/dwarf.h"
 #include "lldb/Utility/RegularExpression.h"
 #include "lldb/lldb-defines.h"
 
 #include "DWARFDefines.h"
 #include "DWARFFormValue.h"
 #include "NameToDIE.h"
 
-class SymbolFileDWARF;
-class DWARFCompileUnit;
-class DWARFDebugInfoEntry;
-
 class DWARFMappedHash {
 public:
   enum AtomType : uint16_t {
     eAtomTypeNULL = 0u,
     eAtomTypeDIEOffset = 1u, // DIE offset, check form for encoding
     eAtomTypeCUOffset = 2u,  // DIE offset of the compiler unit header that
                              // contains the item in question
     eAtomTypeTag = 3u, // DW_TAG_xxx value, should be encoded as DW_FORM_data1