[AST] Construct Capture objects before use

[vitalybuka]

Summary

Msan reports https://reviews.llvm.org/P8308
The reason is if PointerIntPair is not properly
constructed, setPointer uses Info::updatePointer
on uninitialized value.

[vitalybuka]

ping, any concerns regarding this patch?
i want to make MSAN stricter on our bot by reducing -O level

clang/include/clang/AST/Stmt.h

public:
   };
 
   /// Describes the capture of either a variable, or 'this', or
   /// variable-length array type.
   class Capture {
     llvm::PointerIntPair<VarDecl *, 2, VariableCaptureKind> VarAndKind;
     SourceLocation Loc;
 
+    Capture() = default;
+
   public:
     friend class ASTStmtReader;
+    friend class CapturedStmt;
 
     /// Create a new capture.
     ///
     /// \param Loc The source location associated with this capture.
     ///
     /// \param Kind The kind of capture (this, ByRef, ...).
     ///
     /// \param Var The variable being captured, or null if capturing this.

clang/lib/AST/Stmt.cpp

CapturedStmt::CapturedStmt(Stmt *S, CapturedRegionKind Kind,
   Capture *Buffer = getStoredCaptures();
   std::copy(Captures.begin(), Captures.end(), Buffer);
 }
 
 CapturedStmt::CapturedStmt(EmptyShell Empty, unsigned NumCaptures)
   : Stmt(CapturedStmtClass, Empty), NumCaptures(NumCaptures),
     CapDeclAndKind(nullptr, CR_Default) {
   getStoredStmts()[NumCaptures] = nullptr;
+
+  // Construct default capture objects.
+  Capture *Buffer = getStoredCaptures();
+  for (unsigned I = 0, N = NumCaptures; I != N; ++I)
+    new (Buffer++) Capture();
 }
 
 CapturedStmt *CapturedStmt::Create(const ASTContext &Context, Stmt *S,
                                    CapturedRegionKind Kind,
                                    ArrayRef<Capture> Captures,
                                    ArrayRef<Expr *> CaptureInits,
                                    CapturedDecl *CD,
                                    RecordDecl *RD) {