[clang] [Multilib] Custom flags YAML parsing (PR #110657)

[Simon Tatham via cfe-commits]

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@@ -101,6 +101,25 @@ class Multilib {
 
 raw_ostream &operator<<(raw_ostream &OS, const Multilib &M);
 
+namespace custom_flag {
+struct CustomFlagDeclaration;
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statham-arm wrote:

Seems a bit verbose to me: all references to these types end up looking like `custom_flag::CustomFlagFoo`, repeating the words 'custom flag' twice. I'd have thought that putting things inside a namespace called `custom_flag` was so that you _don't_ need to put the words at the start of the identifier names too!

https://github.com/llvm/llvm-project/pull/110657