[NFC][Clang] Fix some comments in clang

Applying post commit comment suggestions from https://reviews.llvm.org/D114025

clang/include/clang/Analysis/CFG.h

@@ -515,7 +515,7 @@ public:
     /// of the most derived class while we're in the base class.
     VirtualBaseBranch,
 
-    /// Number of different kinds, for validity checks. We subtract 1 so that
+    /// Number of different kinds, for assertions. We subtract 1 so that
     /// to keep receiving compiler warnings when we don't cover all enum values
     /// in a switch.
     NumKindsMinusOne = VirtualBaseBranch

clang/lib/Sema/SemaChecking.cpp

@@ -5546,8 +5546,8 @@ ExprResult Sema::BuildAtomicExpr(SourceRange CallRange, SourceRange ExprRange,
 
   // For an arithmetic operation, the implied arithmetic must be well-formed.
   if (Form == Arithmetic) {
-    // GCC does not enforce these rules for GNU atomics, but we do, because if
-    // we didn't it would be very confusing. FIXME:  For whom? How so?
+    // GCC does not enforce these rules for GNU atomics, but we do to help catch
+    // trivial type errors.
     auto IsAllowedValueType = [&](QualType ValType) {
       if (ValType->isIntegerType())
         return true;
@@ -5588,8 +5588,9 @@ ExprResult Sema::BuildAtomicExpr(SourceRange CallRange, SourceRange ExprRange,
   if (!IsC11 && !AtomTy.isTriviallyCopyableType(Context) &&
       !AtomTy->isScalarType()) {
     // For GNU atomics, require a trivially-copyable type. This is not part of
-    // the GNU atomics specification, but we enforce it, because if we didn't it
-    // would be very confusing. FIXME:  For whom? How so?
+    // the GNU atomics specification but we enforce it for consistency with
+    // other atomics which generally all require a trivially-copyable type. This
+    // is because atomics just copy bits.
     Diag(ExprRange.getBegin(), diag::err_atomic_op_needs_trivial_copy)
         << Ptr->getType() << Ptr->getSourceRange();
     return ExprError();

clang/lib/Sema/SemaExprCXX.cpp

@@ -1508,8 +1508,9 @@ Sema::BuildCXXTypeConstructExpr(TypeSourceInfo *TInfo,
   }
 
   // Only construct objects with object types.
-  // There doesn't seem to be an explicit rule for this but functions are
-  // not objects, so they cannot take initializers.
+  // The standard doesn't explicitly forbid function types here, but that's an
+  // obvious oversight, as there's no way to dynamically construct a function
+  // in general.
   if (Ty->isFunctionType())
     return ExprError(Diag(TyBeginLoc, diag::err_init_for_function_type)
                        << Ty << FullRange);