This PR optimizes the input iterator overload of `assign(_InputIterator,
_InputIterator)` in `std::vector<_Tp, _Allocator>` by directly assigning
to already initialized memory, rather than first destroying existing
elements and then constructing new ones. By eliminating unnecessary
destruction and construction, the proposed algorithm enhances the
performance by up to 2x for trivial element types (e.g.,
`std::vector<int>`), up to 2.6x for non-trivial element types like
`std::vector<std::string>`, and up to 3.4x for more complex non-trivial
types (e.g., `std::vector<std::vector<int>>`).
### Google Benchmarks
Benchmark tests (`libcxx/test/benchmarks/vector_operations.bench.cpp`)
were conducted for the `assign()` implementations before and after this
patch. The tests focused on trivial element types like
`std::vector<int>`, and non-trivial element types such as
`std::vector<std::string>` and `std::vector<std::vector<int>>`.
#### Before
```
-------------------------------------------------------------------------------------------------
Benchmark Time CPU Iterations
-------------------------------------------------------------------------------------------------
BM_AssignInputIterIter/vector_int/1024/1024 1157 ns 1169 ns 608188
BM_AssignInputIterIter<32>/vector_string/1024/1024 14559 ns 14710 ns 47277
BM_AssignInputIterIter<32>/vector_vector_int/1024/1024 26846 ns 27129 ns 25925
```
#### After
```
-------------------------------------------------------------------------------------------------
Benchmark Time CPU Iterations
-------------------------------------------------------------------------------------------------
BM_AssignInputIterIter/vector_int/1024/1024 561 ns 566 ns 1242251
BM_AssignInputIterIter<32>/vector_string/1024/1024 5604 ns 5664 ns 128365
BM_AssignInputIterIter<32>/vector_vector_int/1024/1024 7927 ns 8012 ns 88579
```
In particular, test everything with both a normal and a min_allocator,
add tests for a few corner cases and add tests with types that are
trivially relocatable. Also add tests that count the number of
assignments performed by vector::erase, since that is mandated by the
Standard.
This patch is a preparation for optimizing vector::erase.
This patch fixes warnings and errors that come up when running the
benchmarks as part of the test suite. It also adds the necessary Lit
annotations to make it pass in various configurations and increases the
portability of the benchmarks.
This is an intermediate and fairly mechanical step towards unifying the
benchmarks with the rest of the test suite. Moving this around requires
a few changes, notably making sure we don't throw a wrench into the
discovery process of the normal test suite. This won't be a problem
anymore once benchmarks are taken into account by the test setup out of
the box.
