[BinaryFormat] Remove redundant declarations (NFC) (llvm#166316)

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

Once we remove the redundant declarations, Minidump.cpp becomes
effectively empty, so this patch removes the file.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/BinaryFormat/CMakeLists.txt

@@ -6,7 +6,6 @@ add_llvm_component_library(LLVMBinaryFormat
   ELF.cpp
   MachO.cpp
   Magic.cpp
-  Minidump.cpp
   MsgPackDocument.cpp
   MsgPackDocumentYAML.cpp
   MsgPackReader.cpp