[Clang][NFC] Avoid opening namespace std (llvm#95470)

Never opening `namespace std` avoids even the possibility of introducing
new symbols as well as making the code a bit shorter by removing
unnecessary boiler plate.

Author: philnik777

clang/include/clang/Format/Format.h

@@ -5430,9 +5430,7 @@ bool isClangFormatOff(StringRef Comment);
 } // end namespace format
 } // end namespace clang
 
-namespace std {
 template <>
-struct is_error_code_enum<clang::format::ParseError> : std::true_type {};
-} // namespace std
+struct std::is_error_code_enum<clang::format::ParseError> : std::true_type {};
 
 #endif // LLVM_CLANG_FORMAT_FORMAT_H

clang/include/clang/Frontend/PrecompiledPreamble.h

@@ -256,9 +256,7 @@ class BuildPreambleErrorCategory final : public std::error_category {
 std::error_code make_error_code(BuildPreambleError Error);
 } // namespace clang
 
-namespace std {
 template <>
-struct is_error_code_enum<clang::BuildPreambleError> : std::true_type {};
-} // namespace std
+struct std::is_error_code_enum<clang::BuildPreambleError> : std::true_type {};
 
 #endif

clang/include/clang/Frontend/SerializedDiagnosticReader.h

@@ -128,11 +128,8 @@ class SerializedDiagnosticReader {
 } // namespace serialized_diags
 } // namespace clang
 
-namespace std {
-
 template <>
-struct is_error_code_enum<clang::serialized_diags::SDError> : std::true_type {};
-
-} // namespace std
+struct std::is_error_code_enum<clang::serialized_diags::SDError>
+    : std::true_type {};
 
 #endif // LLVM_CLANG_FRONTEND_SERIALIZEDDIAGNOSTICREADER_H

clang/lib/StaticAnalyzer/Checkers/BlockInCriticalSectionChecker.cpp

@@ -202,21 +202,19 @@ class BlockInCriticalSectionChecker : public Checker<check::PostCall> {
 
 REGISTER_LIST_WITH_PROGRAMSTATE(ActiveCritSections, CritSectionMarker)
 
-namespace std {
 // Iterator traits for ImmutableList data structure
 // that enable the use of STL algorithms.
 // TODO: Move these to llvm::ImmutableList when overhauling immutable data
 // structures for proper iterator concept support.
 template <>
-struct iterator_traits<
+struct std::iterator_traits<
     typename llvm::ImmutableList<CritSectionMarker>::iterator> {
   using iterator_category = std::forward_iterator_tag;
   using value_type = CritSectionMarker;
   using difference_type = std::ptrdiff_t;
   using reference = CritSectionMarker &;
   using pointer = CritSectionMarker *;
 };
-} // namespace std
 
 std::optional<MutexDescriptor>
 BlockInCriticalSectionChecker::checkDescriptorMatch(const CallEvent &Call,