Added an AST matcher for declarations that are in the `std` namespace

[aaron.ballman]

You should also update Registry.cpp to add this to clang-query and you should also regenerate the documentation by running clang\docs\tools\dump_ast_matchers.py.

clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp

2079 ↗

(On Diff #197925)

Can you also add a negative test case for:

namespace foo {
namespace std {
  void bar(); // isInStdNamespace() is false here, right?
}
}

[alexfh]

Please regenerate the HTML docs using clang/docs/tools/dump_ast_matchers.py.

[gribozavr]

Regenerated documentation and added one more test

[gribozavr]

clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp

2079 ↗

(On Diff #197925)

Done.

[aaron.ballman]

LGTM!

clang-tidy/bugprone/InaccurateEraseCheck.cpp

 #include "clang/Lex/Lexer.h"
 
 using namespace clang::ast_matchers;
 
 namespace clang {
 namespace tidy {
 namespace bugprone {
 
-namespace {
-AST_MATCHER(Decl, isInStdNamespace) { return Node.isInStdNamespace(); }
-}
-
 void InaccurateEraseCheck::registerMatchers(MatchFinder *Finder) {
   // Only register the matchers for C++; the functionality currently does not
   // provide any benefit to other languages, despite being benign.
   if (!getLangOpts().CPlusPlus)
     return;
 
   const auto EndCall =
       callExpr(