[ADT] Use a constexpr function in SmallVector (NFC)

SmallVector uses CalculateSmallVectorDefaultInlinedElements to compute
the default number of inlined elements.  This patch converts the
struct to a constexpr function.

Author: kazutakahirata

llvm/include/llvm/ADT/SmallVector.h

@@ -1122,20 +1122,18 @@ template <typename T> struct alignas(T) SmallVectorStorage<T, 0> {};
 /// `sizeof(SmallVector<T, 0>)`.
 template <typename T, unsigned N> class LLVM_GSL_OWNER SmallVector;
 
-/// Helper class for calculating the default number of inline elements for
+/// Helper function for calculating the default number of inline elements for
 /// `SmallVector<T>`.
-///
-/// This should be migrated to a constexpr function when our minimum
-/// compiler support is enough for multi-statement constexpr functions.
-template <typename T> struct CalculateSmallVectorDefaultInlinedElements {
+template <typename T>
+constexpr size_t calculateSmallVectorDefaultInlinedElements() {
   // Parameter controlling the default number of inlined elements
   // for `SmallVector<T>`.
   //
   // The default number of inlined elements ensures that
   // 1. There is at least one inlined element.
   // 2. `sizeof(SmallVector<T>) <= kPreferredSmallVectorSizeof` unless
   // it contradicts 1.
-  static constexpr size_t kPreferredSmallVectorSizeof = 64;
+  constexpr size_t kPreferredSmallVectorSizeof = 64;
 
   // static_assert that sizeof(T) is not "too big".
   //
@@ -1168,12 +1166,11 @@ template <typename T> struct CalculateSmallVectorDefaultInlinedElements {
 
   // Discount the size of the header itself when calculating the maximum inline
   // bytes.
-  static constexpr size_t PreferredInlineBytes =
+  constexpr size_t PreferredInlineBytes =
       kPreferredSmallVectorSizeof - sizeof(SmallVector<T, 0>);
-  static constexpr size_t NumElementsThatFit = PreferredInlineBytes / sizeof(T);
-  static constexpr size_t value =
-      NumElementsThatFit == 0 ? 1 : NumElementsThatFit;
-};
+  constexpr size_t NumElementsThatFit = PreferredInlineBytes / sizeof(T);
+  return NumElementsThatFit == 0 ? 1 : NumElementsThatFit;
+}
 
 /// This is a 'vector' (really, a variable-sized array), optimized
 /// for the case when the array is small.  It contains some number of elements
@@ -1192,7 +1189,7 @@ template <typename T> struct CalculateSmallVectorDefaultInlinedElements {
 ///
 /// \see https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h
 template <typename T,
-          unsigned N = CalculateSmallVectorDefaultInlinedElements<T>::value>
+          unsigned N = calculateSmallVectorDefaultInlinedElements<T>()>
 class LLVM_GSL_OWNER SmallVector : public SmallVectorImpl<T>,
                                    SmallVectorStorage<T, N> {
 public: