[lldb][NFC] Remove ClangASTContext::GetUniqueNamespaceDeclaration overload

This overload is only used in one place and having static overloads for
all methods that only do an additional clang::ASTContext -> ClangASTContext
conversion is just not sustainable.
diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionDeclMap.cpp b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionDeclMap.cpp
index 35a50c8..bbc0312 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionDeclMap.cpp
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionDeclMap.cpp
@@ -1101,13 +1101,15 @@
                                    : CompilerDeclContext();
 
       if (frame_decl_context) {
-        ClangASTContext *ast = llvm::dyn_cast_or_null<ClangASTContext>(
+        ClangASTContext *frame_ast = llvm::dyn_cast_or_null<ClangASTContext>(
             frame_decl_context.GetTypeSystem());
 
-        if (ast) {
+        ClangASTContext *map_ast =
+            ClangASTContext::GetASTContext(m_ast_context);
+        if (frame_ast && map_ast) {
           clang::NamespaceDecl *namespace_decl =
-              ClangASTContext::GetUniqueNamespaceDeclaration(
-                  m_ast_context, name.GetCString(), nullptr);
+              map_ast->GetUniqueNamespaceDeclaration(name.GetCString(),
+                                                     nullptr);
           if (namespace_decl) {
             context.AddNamedDecl(namespace_decl);
             clang::DeclContext *clang_decl_ctx =