[Clang][AST][NFC] Add assertion on Init to CompoundLiteralExpr

Static analysis complained that:

  child_range(&Init, &Init+1);

in the children member function was potentially out of bounds. This is false b/c
it is forming an iterator range but it would be invalid if Init was a nullptr.

I add an assertion in the constructor for this and remove to FIXME checks that
are related to this. I checked the various usages and we always valid the
argument is not nullptr.

Author: shafik

clang/include/clang/AST/Expr.h

@@ -3548,6 +3548,7 @@ class CompoundLiteralExpr : public Expr {
                       QualType T, ExprValueKind VK, Expr *init, bool fileScope)
       : Expr(CompoundLiteralExprClass, T, VK, OK_Ordinary),
         LParenLoc(lparenloc), TInfoAndScope(tinfo, fileScope), Init(init) {
+    assert(Init && "Init is a nullptr");
     setDependence(computeDependence(this));
   }
 
@@ -3577,17 +3578,11 @@ class CompoundLiteralExpr : public Expr {
   APValue &getStaticValue() const;
 
   SourceLocation getBeginLoc() const LLVM_READONLY {
-    // FIXME: Init should never be null.
-    if (!Init)
-      return SourceLocation();
     if (LParenLoc.isInvalid())
       return Init->getBeginLoc();
     return LParenLoc;
   }
   SourceLocation getEndLoc() const LLVM_READONLY {
-    // FIXME: Init should never be null.
-    if (!Init)
-      return SourceLocation();
     return Init->getEndLoc();
   }