[Clang][AST] Move NamespaceDecl bits to DeclContext (#98567)

Currently, `NamespaceDecl` has a member `AnonOrFirstNamespaceAndFlags`
which stores a few pieces of data:
- a bit indicating whether the namespace was declared `inline`, and
- a bit indicating whether the namespace was declared as a
_nested-namespace-definition_, and
- a pointer a `NamespaceDecl` that either stores:
- a pointer to the first declaration of that namespace if the
declaration is no the first declaration, or
- a pointer to the unnamed namespace that inhabits the namespace
otherwise.

`Redeclarable` already stores a pointer to the first declaration of an
entity, so it's unnecessary to store this in `NamespaceDecl`.
`DeclContext` has 8 bytes in which various bitfields can be stored for a
declaration, so it's not necessary to store these in `NamespaceDecl`
either. We only need to store a pointer to the unnamed namespace that
inhabits the first declaration of a namespace. This patch moves the two
bits currently stored in `NamespaceDecl` to `DeclContext`, and only
stores a pointer to the unnamed namespace that inhabits a namespace in
the first declaration of that namespace. Since `getOriginalNamespace`
always returns the same `NamespaceDecl` as `getFirstDecl`, this function
is removed to avoid confusion.

2 files changed, 2 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index 8fea7b0..88d4732 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -6864,7 +6864,7 @@ void SemaCodeCompletion::CodeCompleteNamespaceDecl(Scope *S) {
              NS(Ctx->decls_begin()),
          NSEnd(Ctx->decls_end());
          NS != NSEnd; ++NS)
-      OrigToLatest[NS->getOriginalNamespace()] = *NS;
+      OrigToLatest[NS->getFirstDecl()] = *NS;
 
     // Add the most recent definition (or extended definition) of each
     // namespace to the list of results.
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 7851c5d..7a6a645 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -2325,7 +2325,7 @@ static bool LookupQualifiedNameInUsingDirectives(Sema &S, LookupResult &R,
   // We have already looked into the initial namespace; seed the queue
   // with its using-children.
   for (auto *I : StartDC->using_directives()) {
-    NamespaceDecl *ND = I->getNominatedNamespace()->getOriginalNamespace();
+    NamespaceDecl *ND = I->getNominatedNamespace()->getFirstDecl();
     if (S.isVisible(I) && Visited.insert(ND).second)
       Queue.push_back(ND);
   }