[clang-format] Use std::iota and reserve. NFC.

This way we have at most 1 allocation even if the number of includes is greater than the on-stack size of the small vector.

clang/lib/Format/Format.cpp

@@ -44,6 +44,7 @@
 #include <algorithm>
 #include <memory>
 #include <mutex>
+#include <numeric>
 #include <string>
 #include <unordered_map>
 
@@ -2521,8 +2522,8 @@ static void sortCppIncludes(const FormatStyle &Style,
   if (!affectsRange(Ranges, IncludesBeginOffset, IncludesEndOffset))
     return;
   SmallVector<unsigned, 16> Indices;
-  for (unsigned i = 0, e = Includes.size(); i != e; ++i)
-    Indices.push_back(i);
+  Indices.resize(Includes.size());
+  std::iota(Indices.begin(), Indices.end(), 0);
 
   if (Style.SortIncludes == FormatStyle::SI_CaseInsensitive) {
     llvm::stable_sort(Indices, [&](unsigned LHSI, unsigned RHSI) {