Recommit "[Sema] Fix null pointer dereference handleAlwaysInlineAttr."

Remove use of constexpr if that failed on the build bots.

Original commit message:

It's possible for `getCalleeDecl()` to return a null pointer.

This was encountered by a user of our downstream compiler.

The case involved a DependentScopeDeclRefExpr.

Since this seems to only be for a warning diagnostic, I skipped
the diagnostic check if it returned null. But mabye there's a
different way to fix this.

clang/lib/Sema/SemaStmtAttr.cpp

@@ -233,7 +233,8 @@ static Attr *handleNoInlineAttr(Sema &S, Stmt *St, const ParsedAttr &A,
 
   for (const auto *CallExpr : CEF.getCallExprs()) {
     const Decl *Decl = CallExpr->getCalleeDecl();
-    if (Decl->hasAttr<AlwaysInlineAttr>() || Decl->hasAttr<FlattenAttr>())
+    if (Decl &&
+        (Decl->hasAttr<AlwaysInlineAttr>() || Decl->hasAttr<FlattenAttr>()))
       S.Diag(St->getBeginLoc(), diag::warn_function_stmt_attribute_precedence)
           << A << (Decl->hasAttr<AlwaysInlineAttr>() ? 0 : 1);
   }
@@ -259,7 +260,7 @@ static Attr *handleAlwaysInlineAttr(Sema &S, Stmt *St, const ParsedAttr &A,
 
   for (const auto *CallExpr : CEF.getCallExprs()) {
     const Decl *Decl = CallExpr->getCalleeDecl();
-    if (Decl->hasAttr<NoInlineAttr>() || Decl->hasAttr<FlattenAttr>())
+    if (Decl && (Decl->hasAttr<NoInlineAttr>() || Decl->hasAttr<FlattenAttr>()))
       S.Diag(St->getBeginLoc(), diag::warn_function_stmt_attribute_precedence)
           << A << (Decl->hasAttr<NoInlineAttr>() ? 2 : 1);
   }

clang/test/Sema/attr-alwaysinline.cpp

@@ -25,3 +25,16 @@ void foo() {
 }
 
 [[clang::always_inline]] static int i = bar(); // expected-warning {{'always_inline' attribute only applies to functions and statements}}
+
+// This used to crash the compiler.
+template<int D>
+int foo(int x) {
+  [[clang::always_inline]] return foo<D-1>(x + 1);
+}
+
+// FIXME: This should warn that always_inline statement attribute has higher
+// precedence than the noinline function attribute.
+template<int D> [[gnu::noinline]]
+int bar(int x) {
+  [[clang::always_inline]] return bar<D-1>(x + 1);
+}

clang/test/Sema/attr-noinline.cpp

@@ -25,3 +25,16 @@ void foo() {
 }
 
 [[clang::noinline]] static int i = bar(); // expected-warning {{'noinline' attribute only applies to functions and statements}}
+
+// This used to crash the compiler.
+template<int D>
+int foo(int x) {
+  [[clang::noinline]] return foo<D-1>(x + 1);
+}
+
+// FIXME: This should warn that noinline statement attribute has higher
+// precedence than the always_inline function attribute.
+template<int D> [[clang::always_inline]]
+int bar(int x) {
+  [[clang::noinline]] return bar<D-1>(x + 1);
+}