[ELF] Introduce getValue() for ELF Symbols

[davide]

Summary

COFF has a similar function, because linkers, or tool in general, need it. At least, my ELF backend rewrite does.

[• rafael]

LGTM.

[• rafael]

Side question, why do we have Elf_Sym_Base and Elf_Sym_Impl?

include/llvm/Object/ELFTypes.h

struct Elf_Sym_Base<ELFType<TargetEndianness, true>> {
   Elf_Xword st_size;      // Size of the symbol
 };
 
 template <class ELFT>
 struct Elf_Sym_Impl : Elf_Sym_Base<ELFT> {
   using Elf_Sym_Base<ELFT>::st_info;
   using Elf_Sym_Base<ELFT>::st_shndx;
   using Elf_Sym_Base<ELFT>::st_other;
+  using Elf_Sym_Base<ELFT>::st_value;
 
   // These accessors and mutators correspond to the ELF32_ST_BIND,
   // ELF32_ST_TYPE, and ELF32_ST_INFO macros defined in the ELF specification:
   unsigned char getBinding() const { return st_info >> 4; }
   unsigned char getType() const { return st_info & 0x0f; }
+  uint64_t getValue() const { return st_value; }
   void setBinding(unsigned char b) { setBindingAndType(b, getType()); }
   void setType(unsigned char t) { setBindingAndType(getBinding(), t); }
   void setBindingAndType(unsigned char b, unsigned char t) {
     st_info = (b << 4) + (t & 0x0f);
   }
 
   /// Access to the STV_xxx flag stored in the first two bits of st_other.
   /// STV_DEFAULT: 0