[llvm] Reduce memory footprint of Debug metadata nodes (#71227)

Using a combination of reordering fields and using empty SubclassData32
/ SubclassData1, it's possible to improve the size of data structures
used to store debug info in the IR:

Before:

DILexicalBlock: 24
DILexicalBlockFile: 24
DIModule: 24
DITemplateParameter: 24
DICommonBlock: 24
DIMacro: 24
DICompileUnit: 56
DIType: 48
DINamespace: 24
DIVariable: 24
DIGlobalVariable: 32
DILocalVariable: 32
DILabel: 24

After:

DILexicalBlock: 24
DILexicalBlockFile: 16
DIModule: 16
DITemplateParameter: 16
DICommonBlock: 16
DIMacro: 16
DICompileUnit: 48
DIType: 40
DINamespace: 16
DIVariable: 24
DIGlobalVariable: 24
DILocalVariable: 32
DILabel: 16