[clang] [clang-format] Remove YAML hack to emit a BasedOnStyle comment (PR #89228)

[Jannik Silvanus via cfe-commits]

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@@ -807,12 +807,18 @@ template <> struct MappingTraits<FormatStyle> {
         FormatStyle PredefinedStyle;
         if (getPredefinedStyle(StyleName, Style.Language, &PredefinedStyle) &&
             Style == PredefinedStyle) {
-          IO.mapOptional("# BasedOnStyle", StyleName);
+          // For convenience, emit the info which style this matches. However,
+          // setting BasedOnStyle will override all other keys when importing,
+          // so we set a helper key that is ignored when importing.
+          // Ideally, we'd want a YAML comment here, but that's not supported.
+          IO.mapOptional("OrigBasedOnStyle", StyleName);
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jasilvanus wrote:

> I think clang-format will baff if there is styles in the .clang-format file we don't understand, its very unforgiving of new styles

This will set a key that is never read just for reference, as the old comment did. 

> Why is using # not a valid YAML comment?

Using `#` in the textual yaml is a valid comment and perfectly fine. 
However, here we are using a function to set a key named `# BasedOnStyle`, which
just happened to emit a correct YAML comment because the key is not properly quoted/escaped.

This breaks when fixing key quoting. 

https://github.com/llvm/llvm-project/pull/89228