[BinaryFormat] Remove redundant declarations (NFC) (llvm#166010)

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/BinaryFormat/Dwarf.cpp

@@ -1076,10 +1076,3 @@ StringRef (*const llvm::dwarf::EnumTraits<LineNumberOps>::StringFn)(unsigned) =
     LNStandardString;
 StringRef (*const llvm::dwarf::EnumTraits<Index>::StringFn)(unsigned) =
     IndexString;
-
-constexpr char llvm::dwarf::EnumTraits<Attribute>::Type[];
-constexpr char llvm::dwarf::EnumTraits<Form>::Type[];
-constexpr char llvm::dwarf::EnumTraits<Index>::Type[];
-constexpr char llvm::dwarf::EnumTraits<Tag>::Type[];
-constexpr char llvm::dwarf::EnumTraits<LineNumberOps>::Type[];
-constexpr char llvm::dwarf::EnumTraits<LocationAtom>::Type[];