[C] Diagnose declarations hidden in C++ (#137368)

This introduces a new diagnostic, -Wc++-hidden-decl, which is grouped
under -Wc++-compat, that diagnoses declarations which are valid in C but
invalid in C++ due to the type being at the wrong scope. e.g.,
```
  struct S {
    struct T {
      int x;
    } t;
  };

  struct T t; // Valid C, invalid C++
```
This is implementing the other half of #21898

1 files changed, 12 insertions, 1 deletions

diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index 1afda9a..fead99c 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -1422,7 +1422,18 @@ bool DeclContext::Encloses(const DeclContext *DC) const {
     return getPrimaryContext()->Encloses(DC);
 
   for (; DC; DC = DC->getParent())
-    if (!isa<LinkageSpecDecl>(DC) && !isa<ExportDecl>(DC) &&
+    if (!isa<LinkageSpecDecl, ExportDecl>(DC) &&
+        DC->getPrimaryContext() == this)
+      return true;
+  return false;
+}
+
+bool DeclContext::LexicallyEncloses(const DeclContext *DC) const {
+  if (getPrimaryContext() != this)
+    return getPrimaryContext()->LexicallyEncloses(DC);
+
+  for (; DC; DC = DC->getLexicalParent())
+    if (!isa<LinkageSpecDecl, ExportDecl>(DC) &&
         DC->getPrimaryContext() == this)
       return true;
   return false;