[docs] Fix doxygen comments wrongly attached to the clang namespace

[nicolas17]

Summary

Looking at the Doxygen-generated documentation for the clang namespace
currently shows several random comments from different parts of the
codebase. These are caused by:

• File doc comments that aren't marked with \file, so they're attached to the next declaration, which is usually "namespace clang {".
• Class doc comments placed before the namespace rather than before the class.

This commit fixes these comments. The generated doxygen documentation now
has proper docs for several classes and files, and the docs for the clang
namespace is now empty.

[thakis]

lgtm, landing

clang/include/clang/AST/ExternalASTSource.h

public:
   void *getOpaqueValue() { return Value.getOpaqueValue(); }
   static LazyGenerationalUpdatePtr getFromOpaqueValue(void *Ptr) {
     return LazyGenerationalUpdatePtr(ValueType::getFromOpaqueValue(Ptr));
   }
 };
 
 } // namespace clang
 
-/// Specialize PointerLikeTypeTraits to allow LazyGenerationalUpdatePtr to be
-/// placed into a PointerUnion.
 namespace llvm {
 
+/// Specialize PointerLikeTypeTraits to allow LazyGenerationalUpdatePtr to be
+/// placed into a PointerUnion.
 template<typename Owner, typename T,
          void (clang::ExternalASTSource::*Update)(Owner)>
 struct PointerLikeTypeTraits<
     clang::LazyGenerationalUpdatePtr<Owner, T, Update>> {
   using Ptr = clang::LazyGenerationalUpdatePtr<Owner, T, Update>;
 
   static void *getAsVoidPointer(Ptr P) { return P.getOpaqueValue(); }
   static Ptr getFromVoidPointer(void *P) { return Ptr::getFromOpaqueValue(P); }

clang/include/clang/Analysis/FlowSensitive/DataflowValues.h

 
 #ifndef LLVM_CLANG_ANALYSES_DATAFLOW_VALUES
 #define LLVM_CLANG_ANALYSES_DATAFLOW_VALUES
 
 #include "clang/Analysis/CFG.h"
 #include "clang/Analysis/ProgramPoint.h"
 #include "llvm/ADT/DenseMap.h"
 
+namespace clang {
+
 //===----------------------------------------------------------------------===//
 /// Dataflow Directional Tag Classes.  These are used for tag dispatching
 ///  within the dataflow solver/transfer functions to determine what direction
 ///  a dataflow analysis flows.
 //===----------------------------------------------------------------------===//
 
-namespace clang {
 namespace dataflow {
   struct forward_analysis_tag {};
   struct backward_analysis_tag {};
 } // end namespace dataflow
 
 //===----------------------------------------------------------------------===//
 /// DataflowValues.  Container class to store dataflow values for a CFG.
 //===----------------------------------------------------------------------===//

clang/include/clang/StaticAnalyzer/Core/PathSensitive/LoopUnrolling.h

 //===--- LoopUnrolling.h - Unroll loops -------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
-///
+/// \file
 /// This header contains the declarations of functions which are used to decide
 /// which loops should be completely unrolled and mark their corresponding
 /// CFGBlocks. It is done by tracking a stack of loops in the ProgramState. This
 /// way specific loops can be marked as completely unrolled. For considering a
 /// loop to be completely unrolled it has to fulfill the following requirements:
 /// - Currently only forStmts can be considered.
 /// - The bound has to be known.
 /// - The counter variable has not escaped before/in the body of the loop and
 ///   changed only in the increment statement corresponding to the loop. It also
 ///   has to be initialized by a literal in the corresponding initStmt.
 /// - Does not contain goto, switch and returnStmt.
 ///
-///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_STATICANALYZER_CORE_PATHSENSITIVE_LOOPUNROLLING_H
 #define LLVM_CLANG_STATICANALYZER_CORE_PATHSENSITIVE_LOOPUNROLLING_H
 
 #include "clang/Analysis/CFG.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/ProgramState.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/ExplodedGraph.h"

clang/include/clang/StaticAnalyzer/Core/PathSensitive/LoopWidening.h

 //===--- LoopWidening.h - Widen loops ---------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
-///
+/// \file
 /// This header contains the declarations of functions which are used to widen
 /// loops which do not otherwise exit. The widening is done by invalidating
 /// anything which might be modified by the body of the loop.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_STATICANALYZER_CORE_PATHSENSITIVE_LOOPWIDENING_H
 #define LLVM_CLANG_STATICANALYZER_CORE_PATHSENSITIVE_LOOPWIDENING_H