[Clang] Reduce the size of Decl and classes derived from it (#87361)

[Clang] Reduce the size of Decl and classes derived from it (#87361)

Class | Old size (in bytes) | New size (in bytes)

----------------------------------|---------------------|--------------------
Decl                              | 40                  | 32
AccessSpecDecl                    | 40                  | 40
BlockDecl                         | 128                 | 120
CapturedDecl                      | 88                  | 80
EmptyDecl                         | 40                  | 32
ExportDecl                        | 80                  | 72
ExternCContextDecl                | 72                  | 64
FileScopeAsmDecl                  | 56                  | 48
FriendDecl                        | 64                  | 56
FriendTemplateDecl                | 64                  | 64
ImplicitConceptSpecializationDecl | 40                  | 40
ImportDecl                        | 56                  | 48
LifetimeExtendedTemporaryDecl     | 72                  | 64
LinkageSpecDecl                   | 80                  | 72
NamedDecl                         | 48                  | 40
ObjCPropertyImplDecl              | 96                  | 88
PragmaCommentDecl                 | 40                  | 40
PragmaDetectMismatchDecl          | 48                  | 40
RequiresExprBodyDecl              | 72                  | 64
StaticAssertDecl                  | 64                  | 56
TopLevelStmtDecl                  | 88                  | 80
TranslationUnitDecl               | 104                 | 96
BaseUsingDecl                     | 56                  | 48
UsingDecl                         | 88                  | 80
UsingEnumDecl                     | 72                  | 64
HLSLBufferDecl                    | 96                  | 88
LabelDecl                         | 80                  | 72
NamespaceAliasDecl                | 96                  | 88
NamespaceDecl                     | 112                 | 104
ObjCCompatibleAliasDecl           | 56                  | 48
ObjCContainerDecl                 | 88                  | 80
ObjCMethodDecl                    | 136                 | 128
ObjCPropertyDecl                  | 128                 | 120
TemplateDecl                      | 64                  | 56
BuiltinTemplateDecl               | 72                  | 64
TypeDecl                          | 64                  | 56
UnresolvedUsingIfExistsDecl       | 48                  | 40
UsingDirectiveDecl                | 88                  | 80
UsingPackDecl                     | 64                  | 56
UsingShadowDecl                   | 80                  | 72
ValueDecl                         | 56                  | 48

When parsing libc++'s `<string>` header the used memory is reduced from
42.8MB to 42.5MB.