[clang][NFC] Fix typos/wording in the comments of ConstantExpr.

It is "trailing objects" and "tail-allocated storage".

clang/include/clang/AST/Expr.h

@@ -988,11 +988,11 @@ class ConstantExpr final
     : public FullExpr,
       private llvm::TrailingObjects<ConstantExpr, APValue, uint64_t> {
   static_assert(std::is_same<uint64_t, llvm::APInt::WordType>::value,
-                "this class assumes llvm::APInt::WordType is uint64_t for "
-                "trail-allocated storage");
+                "ConstantExpr assumes that llvm::APInt::WordType is uint64_t "
+                "for tail-allocated storage");
 
 public:
-  /// Describes the kind of result that can be trail-allocated.
+  /// Describes the kind of result that can be tail-allocated.
   enum ResultStorageKind { RSK_None, RSK_Int64, RSK_APValue };
 
 private:

clang/include/clang/AST/Stmt.h

@@ -322,26 +322,26 @@ protected:
 
     unsigned : NumExprBits;
 
-    /// The kind of result that is trail-allocated.
+    /// The kind of result that is tail-allocated.
     unsigned ResultKind : 2;
 
-    /// Kind of Result as defined by APValue::Kind
+    /// The kind of Result as defined by APValue::Kind
     unsigned APValueKind : 4;
 
-    /// When ResultKind == RSK_Int64. whether the trail-allocated integer is
-    /// signed.
+    /// When ResultKind == RSK_Int64, true if the tail-allocated integer is
+    /// unsigned.
     unsigned IsUnsigned : 1;
 
-    /// When ResultKind == RSK_Int64. the BitWidth of the trail-allocated
-    /// integer. 7 bits because it is the minimal number of bit to represent a
-    /// value from 0 to 64 (the size of the trail-allocated number).
+    /// When ResultKind == RSK_Int64. the BitWidth of the tail-allocated
+    /// integer. 7 bits because it is the minimal number of bits to represent a
+    /// value from 0 to 64 (the size of the tail-allocated integer).
     unsigned BitWidth : 7;
 
-    /// When ResultKind == RSK_APValue. Wether the ASTContext will cleanup the
-    /// destructor on the trail-allocated APValue.
+    /// When ResultKind == RSK_APValue, true if the ASTContext will cleanup the
+    /// tail-allocated APValue.
     unsigned HasCleanup : 1;
 
-    /// Whether this ConstantExpr was created for immediate invocation.
+    /// True if this ConstantExpr was created for immediate invocation.
     unsigned IsImmediateInvocation : 1;
   };