[llvm] [ADT] Refactor Bitset to Be More Constexpr-Usable and Add More Member Functions (PR #172062)

[Jiachen Yuan via llvm-commits]

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@@ -67,8 +77,11 @@ class Bitset {
       set(I);
   }
 
-  Bitset &set() {
-    llvm::fill(Bits, -BitWord(0));
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JiachenYuan wrote:

Oh yeah, sorry I misread. This original implementation does seem incorrect. According to [std::bitset](https://en.cppreference.com/w/cpp/utility/bitset/set.html), `set()` without any argument should set all bits to 1.

https://github.com/llvm/llvm-project/pull/172062