[clang-format] Allow empty .clang-format file

[curdeius]

I have stumbled on it myself several times but never really needed a fix.
Out of curiosity, what motivates you to do this change?

[owenpan]

In D123535#3443604, @curdeius wrote:

I have stumbled on it myself several times but never really needed a fix.
Out of curiosity, what motivates you to do this change?

I was playing with adding/removing some options in a .clang-format file that sometimes became empty as a result. I could work around this bug by commenting out the options instead of removing them but decided to fix it because sooner or later someone else will encounter the same problem and file a bug report.

[owenpan]

Updated the affected clang test.

[owenpan]

Updated command line tests.

clang/lib/Format/Format.cpp

 std::error_code parseConfiguration(llvm::MemoryBufferRef Config,
                                    FormatStyle *Style, bool AllowUnknownOptions,
                                    llvm::SourceMgr::DiagHandlerTy DiagHandler,
                                    void *DiagHandlerCtxt) {
   assert(Style);
   FormatStyle::LanguageKind Language = Style->Language;
   assert(Language != FormatStyle::LK_None);
   if (Config.getBuffer().trim().empty())
-    return make_error_code(ParseError::Error);
+    return make_error_code(ParseError::Success);
   Style->StyleSet.Clear();
   std::vector<FormatStyle> Styles;
   llvm::yaml::Input Input(Config, /*Ctxt=*/nullptr, DiagHandler,
                           DiagHandlerCtxt);
   // DocumentListTraits<vector<FormatStyle>> uses the context to get default
   // values for the fields, keys for which are missing from the configuration.
   // Mapping also uses the context to get the language to find the correct
   // base style.

clang/test/Format/style-on-command-line.cpp

 // RUN: clang-format -style="{BasedOnStyle: Google, IndentWidth: 8}" %s | FileCheck -strict-whitespace -check-prefix=CHECK1 %s
 // RUN: clang-format -style="{BasedOnStyle: LLVM, IndentWidth: 7}" %s | FileCheck -strict-whitespace -check-prefix=CHECK2 %s
 // RUN: not clang-format -style="{BasedOnStyle: invalid, IndentWidth: 7}" -fallback-style=LLVM %s 2>&1 | FileCheck -DMSG=%errc_EINVAL -strict-whitespace -check-prefix=CHECK3 %s
 // RUN: not clang-format -style="{lsjd}" %s -fallback-style=LLVM 2>&1 | FileCheck -DMSG=%errc_EINVAL -strict-whitespace -check-prefix=CHECK4 %s
 // RUN: mkdir -p %t
 // RUN: printf "BasedOnStyle: google\nIndentWidth: 5\n" > %t/.clang-format
 // RUN: clang-format -style=file -assume-filename=%t/foo.cpp < %s | FileCheck -strict-whitespace -check-prefix=CHECK5 %s
 // RUN: printf "\n" > %t/.clang-format
-// RUN: not clang-format -style=file -fallback-style=webkit -assume-filename=%t/foo.cpp < %s 2>&1 | FileCheck -strict-whitespace -check-prefix=CHECK6 %s
+// RUN: clang-format -style=file -fallback-style=webkit -assume-filename=%t/foo.cpp < %s 2>&1 | FileCheck -strict-whitespace -check-prefix=CHECK6 %s
 // RUN: rm %t/.clang-format
 // RUN: printf "BasedOnStyle: google\nIndentWidth: 6\n" > %t/_clang-format
 // RUN: clang-format -style=file -assume-filename=%t/foo.cpp < %s | FileCheck -strict-whitespace -check-prefix=CHECK7 %s
 // RUN: clang-format -style="{BasedOnStyle: LLVM, PointerBindsToType: true}" %s | FileCheck -strict-whitespace -check-prefix=CHECK8 %s
 // RUN: clang-format -style="{BasedOnStyle: WebKit, PointerBindsToType: false}" %s | FileCheck -strict-whitespace -check-prefix=CHECK9 %s
 
 // Fallback style tests
 // Test config file with no based style, and fallback style "none", formatting is applied