[AST] Fix a clang crash on an invalid for-range statement.

Summary:
crash stack:

```
llvm-project/clang/lib/AST/ASTContext.cpp:2248: clang::TypeInfo clang::ASTContext::getTypeInfoImpl(const clang::Type *) const: Assertion `!A->getDeducedType().isNull() && "cannot request the size of an undeduced or dependent auto type"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
 #0 0x00000000025bb0bf llvm::sys::PrintStackTrace(llvm::raw_ostream&) llvm-project/llvm/lib/Support/Unix/Signals.inc:564:13
 #1 0x00000000025b92b0 llvm::sys::RunSignalHandlers() llvm-project/llvm/lib/Support/Signals.cpp:69:18
 rust-lang#2 0x00000000025bb535 SignalHandler(int) llvm-project/llvm/lib/Support/Unix/Signals.inc:396:3
 rust-lang#3 0x00007f9ef9298110 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14110)
 rust-lang#4 0x00007f9ef8d72761 raise /build/glibc-M65Gwz/glibc-2.30/signal/../sysdeps/unix/sysv/linux/raise.c:51:1
 rust-lang#5 0x00007f9ef8d5c55b abort /build/glibc-M65Gwz/glibc-2.30/stdlib/abort.c:81:7
 rust-lang#6 0x00007f9ef8d5c42f get_sysdep_segment_value /build/glibc-M65Gwz/glibc-2.30/intl/loadmsgcat.c:509:8
 rust-lang#7 0x00007f9ef8d5c42f _nl_load_domain /build/glibc-M65Gwz/glibc-2.30/intl/loadmsgcat.c:970:34
 rust-lang#8 0x00007f9ef8d6b092 (/lib/x86_64-linux-gnu/libc.so.6+0x34092)
 rust-lang#9 0x000000000458abe0 clang::ASTContext::getTypeInfoImpl(clang::Type const*) const llvm-project/clang/lib/AST/ASTContext.cpp:0:5
```

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81384

clang/lib/Sema/SemaStmt.cpp

@@ -2434,8 +2434,11 @@ StmtResult Sema::BuildCXXForRangeStmt(SourceLocation ForLoc,
     QualType RangeType = Range->getType();
 
     if (RequireCompleteType(RangeLoc, RangeType,
-                            diag::err_for_range_incomplete_type))
+                            diag::err_for_range_incomplete_type)) {
+      if (LoopVar->getType()->isUndeducedType())
+        LoopVar->setInvalidDecl();
       return StmtError();
+    }
 
     // Build auto __begin = begin-expr, __end = end-expr.
     // Divide by 2, since the variables are in the inner scope (loop body).

clang/test/SemaCXX/for-range-crash.cpp

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+
+template <typename>
+class Bar {
+  Bar<int> *variables_to_modify;
+  foo() { // expected-error {{C++ requires a type specifier for all declarations}}
+    for (auto *c : *variables_to_modify)
+      delete c;
+  }
+};