[clang-format] Add a default format style that can be used by users of `getStyle`

[ioeric]

Summary

Tools that reformat code often call getStyle to decide the format style
to use on a certain source file. In practice, "file" style is widely used. As a
result, many tools hardcode "file" when calling getStyle, which makes it hard
to control the default style in tools across a codebase when needed. This change
introduces a DefaultFormatStyle constant (default to "file" in upstream), which
can be modified downstream if wanted, so that all users/tools built from the same
source tree can have a consistent default format style.

This also adds an DefaultFallbackStyle that is recommended to be used by tools and can be modified downstream.

[sammccall]

I think this should work great for us, and hopefully helps other downstream users too.

(@djasper: the plan is to introduce a new style name for our tweaked version of "file", and make it the default)

tools/clang-format/ClangFormat.cpp

67

My only question is if we want to do the same with DefaultFallbackStyle.

Not because we have a use case for changing it right now, but because it's another string that callers of getStyle() have to hard-code. In our internal codebase, some pass "LLVM", some pass "Google", and some make it an option.

[ioeric]

• Add DefaultFallbackStyle

[ioeric]

tools/clang-format/ClangFormat.cpp

67

Can't see a reason not to. This should help get rid of more hardcode.

include/clang/Format/Format.h

 /// \param Style determines specific settings for lexing mode.
 LangOptions getFormattingLangOpts(const FormatStyle &Style = getLLVMStyle());
 
 /// Description to be used for help text for a ``llvm::cl`` option for
 /// specifying format style. The description is closely related to the operation
 /// of ``getStyle()``.
 extern const char *StyleOptionHelpDescription;
 
+/// The suggested format style to use by default. This allows tools using
+/// `getStyle` to have a consistent default style.
+/// Different builds can modify the value to the preferred styles.
+extern const char *DefaultFormatStyle;
+
 /// Construct a FormatStyle based on ``StyleName``.
 ///
 /// ``StyleName`` can take several forms:
 /// * "{<key>: <value>, ...}" - Set specic style parameters.
 /// * "<style name>" - One of the style names supported by
 /// getPredefinedStyle().
 /// * "file" - Load style configuration from a file called ``.clang-format``
 /// located in one of the parent directories of ``FileName`` or the current

lib/Format/Format.cpp

if (Extension.empty() || Extension == ".h") {
       Guesser.process();
       if (Guesser.isObjC())
         return FormatStyle::LK_ObjC;
     }
   }
   return GuessedLanguage;
 }
 
+const char *DefaultFormatStyle = "file";
+
 llvm::Expected<FormatStyle> getStyle(StringRef StyleName, StringRef FileName,
                                      StringRef FallbackStyleName,
                                      StringRef Code, vfs::FileSystem *FS) {
   if (!FS) {
     FS = vfs::getRealFileSystem().get();
   }
   FormatStyle Style = getLLVMStyle();
   Style.Language = guessLanguage(FileName, Code);

tools/clang-format/ClangFormat.cpp

 LineRanges("lines", cl::desc("<start line>:<end line> - format a range of\n"
                              "lines (both 1-based).\n"
                              "Multiple ranges can be formatted by specifying\n"
                              "several -lines arguments.\n"
                              "Can't be used with -offset and -length.\n"
                              "Can only be used with one input file."),
            cl::cat(ClangFormatCategory));
 static cl::opt<std::string>
-    Style("style",
-          cl::desc(clang::format::StyleOptionHelpDescription),
-          cl::init("file"), cl::cat(ClangFormatCategory));
+    Style("style", cl::desc(clang::format::StyleOptionHelpDescription),
+          cl::init(clang::format::DefaultFormatStyle),
+          cl::cat(ClangFormatCategory));
 static cl::opt<std::string>
 FallbackStyle("fallback-style",
               cl::desc("The name of the predefined style used as a\n"
                        "fallback in case clang-format is invoked with\n"
                        "-style=file, but can not find the .clang-format\n"
                        "file to use.\n"
                        "Use -fallback-style=none to skip formatting."),
               cl::init("LLVM"), cl::cat(ClangFormatCategory));
 
 static cl::opt<std::string>