[clang] Fix isInStdNamespace for Decl flagged extern c++ (#81776)

The MSVC STL implementation declares multiple classes using:

```cpp
namespace std {
  extern "C++" class locale {
    ...
  };
}
```

`isInStdNamespace` uses the first DeclContext to check whether a Decl is
inside the `std` namespace. Here, the first DeclContext of the `locale`
Decl is a LinkageSpecDecl so the method will return false.
We need to skip this LinkageSpecDecl to find the first DeclContext of
type Namespace and actually check whether we're in the `std` namespace.

Author: frederic-tingaud-sonarsource

clang/docs/ReleaseNotes.rst

@@ -327,6 +327,8 @@ Fixed Point Support in Clang
 AST Matchers
 ------------
 
+- ``isInStdNamespace`` now supports Decl declared with ``extern "C++"``.
+
 clang-format
 ------------
 

clang/lib/AST/DeclBase.cpp

@@ -402,7 +402,7 @@ bool Decl::isInAnonymousNamespace() const {
 
 bool Decl::isInStdNamespace() const {
   const DeclContext *DC = getDeclContext();
-  return DC && DC->isStdNamespace();
+  return DC && DC->getNonTransparentContext()->isStdNamespace();
 }
 
 bool Decl::isFileContextDecl() const {

clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp

@@ -3637,6 +3637,11 @@ TEST_P(ASTMatchersTest, InStdNamespace) {
                       "  class vector {};"
                       "}",
                       cxxRecordDecl(hasName("vector"), isInStdNamespace())));
+
+  EXPECT_TRUE(matches("namespace std {"
+                      "  extern \"C++\" class vector {};"
+                      "}",
+                      cxxRecordDecl(hasName("vector"), isInStdNamespace())));
 }
 
 TEST_P(ASTMatchersTest, InAnonymousNamespace) {