[clang] SIGSEGV fix at clang::ASTContext::getRawCommentForDeclNoCacheImpl

The `File` might point to an invalid `FileID` when the AST is broken. That leads to clang/clangd crashes while processing comments. The relevant part of the crash is below
```
 #4 0x00007f1d7fbf95bc std::_Rb_tree<unsigned int, std::pair<unsigned int const, clang::RawComment*>, std::_Select1st<std::pair<unsigned int const, clang::RawComment*>>, std::less<unsigned int>, std::allocator<std::pair<unsigned int const
, clang::RawComment*>>>::_M_lower_bound(std::_Rb_tree_node<std::pair<unsigned int const, clang::RawComment*>> const*, std::_Rb_tree_node_base const*, unsigned int const&) const /usr/include/c++/8/bits/stl_tree.h:1911:2
 #5 0x00007f1d7fbf95bc std::_Rb_tree<unsigned int, std::pair<unsigned int const, clang::RawComment*>, std::_Select1st<std::pair<unsigned int const, clang::RawComment*>>, std::less<unsigned int>, std::allocator<std::pair<unsigned int const, clang::RawComment*>>>::lower_bound(unsigned int const&) const /usr/include/c++/8/bits/stl_tree.h:1214:56
 #6 0x00007f1d7fbf95bc std::map<unsigned int, clang::RawComment*, std::less<unsigned int>, std::allocator<std::pair<unsigned int const, clang::RawComment*>>>::lower_bound(unsigned int const&) const /usr/include/c++/8/bits/stl_map.h:1264:36
 #7 0x00007f1d7fbf95bc clang::ASTContext::getRawCommentForDeclNoCacheImpl(clang::Decl const*, clang::SourceLocation, std::map<unsigned int, clang::RawComment*, std::less<unsigned int>, std::allocator<std::pair<unsigned int const, clang::RawComment*>>> const&) const /home/ivanmurashko/local/llvm-project/clang/lib/AST/ASTContext.cpp:226:57
```

The corresponding LIT test that reproduces the crash was also added

Same issue is described at https://bugs.llvm.org/show_bug.cgi?id=49707

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D131675

Author: ivanmurashko

clang/lib/AST/ASTContext.cpp

@@ -298,6 +298,9 @@ RawComment *ASTContext::getRawCommentForDeclNoCache(const Decl *D) const {
     return nullptr;
 
   const FileID File = SourceMgr.getDecomposedLoc(DeclLoc).first;
+  if (!File.isValid()) {
+    return nullptr;
+  }
   const auto CommentsInThisFile = Comments.getCommentsInFile(File);
   if (!CommentsInThisFile || CommentsInThisFile->empty())
     return nullptr;

clang/test/AST/ast-crash-doc.cpp

@@ -0,0 +1,30 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -emit-module -x c++ -fmodules -I %t/Inputs -fmodule-name=aa %t/Inputs/module.modulemap -o %t/aa.pcm
+// RUN: rm %t/Inputs/b.h
+// RUN: not %clang_cc1 -x c++ -Wdocumentation -ast-dump-all -fmodules -I %t/Inputs -fmodule-file=%t/aa.pcm %t/test.cpp | FileCheck %s
+
+//--- Inputs/module.modulemap
+module aa {
+    header "a.h"
+    header "b.h"
+}
+
+//--- Inputs/a.h
+// empty file
+
+//--- Inputs/b.h
+/// test foo @return
+int foo();
+
+
+//--- test.cpp
+#include "a.h"
+
+/// test comment at the primary file
+
+int a = foo();
+
+
+// CHECK: TranslationUnitDecl