[clang][NFC] Refactor `clang::Linkage` (#71049)

This patch introduces a new enumerator `Invalid = 0`, shifting other enumerators by +1. Contrary to how it might sound, this actually affirms status quo of how this enum is stored in `clang::Decl`:
```
  /// If 0, we have not computed the linkage of this declaration.
  /// Otherwise, it is the linkage + 1.
  mutable unsigned CacheValidAndLinkage : 3;
```
This patch makes debuggers to not be mistaken about enumerator stored in this bit-field. It also converts `clang::Linkage` to a scoped enum.

1 files changed, 2 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index cc81a68..35f651b 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -3997,7 +3997,7 @@ TargetMVPriority(const TargetInfo &TI,
 llvm::GlobalValue::LinkageTypes getMultiversionLinkage(CodeGenModule &CGM,
                                                        GlobalDecl GD) {
   const FunctionDecl *FD = cast<FunctionDecl>(GD.getDecl());
-  if (FD->getFormalLinkage() == InternalLinkage)
+  if (FD->getFormalLinkage() == Linkage::Internal)
     return llvm::GlobalValue::InternalLinkage;
   return llvm::GlobalValue::WeakODRLinkage;
 }
@@ -5051,7 +5051,7 @@ void CodeGenModule::MaybeHandleStaticInExternC(const SomeDecl *D,
     return;
 
   // Must have internal linkage and an ordinary name.
-  if (!D->getIdentifier() || D->getFormalLinkage() != InternalLinkage)
+  if (!D->getIdentifier() || D->getFormalLinkage() != Linkage::Internal)
     return;
 
   // Must be in an extern "C" context. Entities declared directly within