Make third_party/llvm-subzero compatible with C++23

This imports the following change from upstream LLVM.
https://github.com/llvm/llvm-project/commit/687bd77e2c26487cba727aacfa7067dd01286be0

Bug: 388070065
Change-Id: Ie337bf08f768f91767a0842f97efe3bd82dfd3bb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/76508
Reviewed-by: Shahbaz Youssefi <syoussefi@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@google.com>
Tested-by: Shahbaz Youssefi <syoussefi@google.com>

third_party/llvm-subzero/include/llvm/ADT/APFloat.h

diff --git a/third_party/llvm-subzero/include/llvm/ADT/APFloat.h b/third_party/llvm-subzero/include/llvm/ADT/APFloat.h
index 0030423..249194f 100644
--- a/third_party/llvm-subzero/include/llvm/ADT/APFloat.h
+++ b/third_party/llvm-subzero/include/llvm/ADT/APFloat.h
@@ -618,21 +618,14 @@
   DoubleAPFloat(DoubleAPFloat &&RHS);
 
   DoubleAPFloat &operator=(const DoubleAPFloat &RHS);
-
-  DoubleAPFloat &operator=(DoubleAPFloat &&RHS) {
-    if (this != &RHS) {
-      this->~DoubleAPFloat();
-      new (this) DoubleAPFloat(std::move(RHS));
-    }
-    return *this;
-  }
+  inline DoubleAPFloat &operator=(DoubleAPFloat &&RHS);
 
   bool needsCleanup() const { return Floats != nullptr; }
 
-  APFloat &getFirst() { return Floats[0]; }
-  const APFloat &getFirst() const { return Floats[0]; }
-  APFloat &getSecond() { return Floats[1]; }
-  const APFloat &getSecond() const { return Floats[1]; }
+  inline APFloat &getFirst();
+  inline const APFloat &getFirst() const;
+  inline APFloat &getSecond();
+  inline const APFloat &getSecond() const;
 
   opStatus add(const DoubleAPFloat &RHS, roundingMode RM);
   opStatus subtract(const DoubleAPFloat &RHS, roundingMode RM);
@@ -1123,6 +1116,27 @@
   return (A.compare(B) == APFloat::cmpLessThan) ? B : A;
 }
 
+// We want the following functions to be available in the header for inlining.
+// We cannot define them inline in the class definition of `DoubleAPFloat`
+// because doing so would instantiate `std::unique_ptr<APFloat[]>` before
+// `APFloat` is defined, and that would be undefined behavior.
+namespace detail {
+
+DoubleAPFloat &DoubleAPFloat::operator=(DoubleAPFloat &&RHS) {
+  if (this != &RHS) {
+    this->~DoubleAPFloat();
+    new (this) DoubleAPFloat(std::move(RHS));
+  }
+  return *this;
+}
+
+APFloat &DoubleAPFloat::getFirst() { return Floats[0]; }
+const APFloat &DoubleAPFloat::getFirst() const { return Floats[0]; }
+APFloat &DoubleAPFloat::getSecond() { return Floats[1]; }
+const APFloat &DoubleAPFloat::getSecond() const { return Floats[1]; }
+
+} // namespace detail
+
 } // namespace llvm
 
 #endif // LLVM_ADT_APFLOAT_H