[clang-format] Fix segfault when printing diagnostic

[rmelotte]

Summary

When clang-format couldn't replace the file to format for some reason,
it would segfault instead of printing a diagnostic message.

A very simple way to reproduce the issue is to create a non-writable
folder containing a file to format, and try to format the file
in-place.

On recent clang versions (11.0.0 for example), the following assert would
show up:
Assertion `getClient() && "DiagnosticClient not set!"' failed.

Set the DiagnosticClient by using a TextDiagnosticPrinter, similarly
to what is done for RewriterTestContext.

---

Here is a very simple reproducer:

mkdir folder
echo "int main(){printf("hello"); return 0;}" > folder/file.c
chmod -w folder
build/bin/clang-format -i folder/file.c
chmod +w folder
rm -rf folder

[MyDeveloperDay]

We specifically dropped clangFrontEnd from clang-format, please don't include it again

[MyDeveloperDay]

see the discussion at D68969: [clang-format] Remove the dependency on frontendD69854: [clang-format] [RELAND] Remove the dependency on frontendD68554: [clang-format] Proposal for clang-format to give compiler style warnings

given the effort to remove frontEnd It would be a shame to simply just bring it back like like that. (it has an impact I believe on binary size etc...)

clang/tools/clang-format/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS support)
 
 add_clang_tool(clang-format
   ClangFormat.cpp
   )
 
 set(CLANG_FORMAT_LIB_DEPS
   clangBasic
   clangFormat
+  clangFrontend
   clangRewrite
   clangToolingCore
   )
 
 clang_target_link_libraries(clang-format
   PRIVATE
   ${CLANG_FORMAT_LIB_DEPS}
   )

clang/tools/clang-format/ClangFormat.cpp

 //===----------------------------------------------------------------------===//
 
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Basic/FileManager.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/Version.h"
 #include "clang/Format/Format.h"
+#include "clang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Rewrite/Core/Rewriter.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/Process.h"
 
 using namespace llvm;
 using clang::tooling::Replacements;
static bool format(StringRef FileName) {
   } else {
     IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> InMemoryFileSystem(
         new llvm::vfs::InMemoryFileSystem);
     FileManager Files(FileSystemOptions(), InMemoryFileSystem);
     DiagnosticsEngine Diagnostics(
         IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs),
         new DiagnosticOptions);
     SourceManager Sources(Diagnostics, Files);
+    TextDiagnosticPrinter DiagnosticPrinter(
+        llvm::outs(), &Diagnostics.getDiagnosticOptions());
+    Diagnostics.setClient(&DiagnosticPrinter, /*ShouldOwnClient=*/false);
     FileID ID = createInMemoryFile(AssumedFileName, Code.get(), Sources, Files,
                                    InMemoryFileSystem.get());
     Rewriter Rewrite(Sources, LangOptions());
     tooling::applyAllReplacements(Replaces, Rewrite);
     if (Inplace) {
       if (Rewrite.overwriteChangedFiles())
         return true;
     } else {