[llvm] Remove redundant declarations (NFC) (llvm#166713)

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/CodeGen/SafeStack.cpp

@@ -196,8 +196,6 @@ class SafeStack {
   bool run();
 };
 
-constexpr Align SafeStack::StackAlignment;
-
 uint64_t SafeStack::getStaticAllocaAllocationSize(const AllocaInst* AI) {
   uint64_t Size = DL.getTypeAllocSize(AI->getAllocatedType());
   if (AI->isArrayAllocation()) {

llvm/lib/Target/BPF/BPFAbstractMemberAccess.cpp

@@ -97,7 +97,6 @@
 #define DEBUG_TYPE "bpf-abstract-member-access"
 
 namespace llvm {
-constexpr StringRef BPFCoreSharedInfo::AmaAttr;
 uint32_t BPFCoreSharedInfo::SeqNum;
 
 Instruction *BPFCoreSharedInfo::insertPassThrough(Module *M, BasicBlock *BB,

llvm/lib/Target/BPF/BPFPreserveDIType.cpp

@@ -27,10 +27,6 @@
 
 #define DEBUG_TYPE "bpf-preserve-di-type"
 
-namespace llvm {
-constexpr StringRef BPFCoreSharedInfo::TypeIdAttr;
-} // namespace llvm
-
 using namespace llvm;
 
 namespace {