[clang] [clang-format] Add option AllowShortRecordOnASingleLine (PR #154580)

[Tomáš Slanina via cfe-commits]

================
@@ -1553,6 +1563,7 @@ FormatStyle getLLVMStyle(FormatStyle::LanguageKind Language) {
   LLVMStyle.AllowShortFunctionsOnASingleLine = FormatStyle::SFS_All;
   LLVMStyle.AllowShortIfStatementsOnASingleLine = FormatStyle::SIS_Never;
   LLVMStyle.AllowShortLambdasOnASingleLine = FormatStyle::SLS_All;
+  LLVMStyle.AllowShortRecordOnASingleLine = FormatStyle::SRS_Never;
----------------
itzexpoexpo wrote:

I'll try to clarify:
```c++
// a.cpp
class foo {};
// Omitting the contentful class to keep the comment shorter
```
Config (when `-style=file` specified, referring to default style otherwise):
```yaml
BasedOnStyle: LLVM
BreakBeforeBraces: Custom
BraceWrapping:
  AfterClass: true
  SplitEmptyRecord: false
```

Before patch:
```c++
$ clang-format a.cpp
class foo {};

$ clang-format a.cpp -style=file:.clang_format
class foo
{};
```
With patch as of latest commit:
```c++
$ clang-format a.cpp
class foo {};

$ clang-format a.cpp -style=file:.clang_format
class foo
{};
```
i.e. feature is entirely opt-in for users, no breaking changes.

Suggested change to make `Never` (default as of latest commit) explicitly disallow merging short records, instead of being treated more like "ignore this option":
```c++
$ clang-format a.cpp
class foo {
};

$ clang-format a.cpp -style=file:.clang-format
class foo
{};
```
(This should be the correct behavior, also same as if #154796 went through)

Suggested change to make `Empty` the default instead:
```c++
$ clang-format a.cpp
class foo {};

$ clang-format a.cpp -style=file:.clang_format
class foo {};
```

Both suggested changes mean that existing code will be formatted differently, I'd like to confirm whether that's okay and you'd like to go through with one or both of the changes?

An alternative I can offer would be to introduce a separate `Ignore` option for `AllowShortRecordOnASingleLine` and make that the default, since no other options use such a value I didn't either, but it would allow implementing `Never` correctly.

https://github.com/llvm/llvm-project/pull/154580