[SmallVector] Add an explicit SmallVector(size_t Size) constructor.

[craig.topper]

Summary

Previously we use the SmallVector(size_t Size, const T& Value) constructor
with a default constructed Value. This will copy construct every element
in the vector, but not all types can be copy constructed.

[dblaikie]

Sounds good - I probably wouldn't mind slightly more in the test, like a check for size or something, but yeah, that'd be of minimal value.

[thurston]

llvm/unittests/ADT/SmallVectorTest.cpp

174–177

This test broke the clang-ppc64-aix build at https://lab.llvm.org/buildbot/#/builders/214/builds/6761/steps/6/logs/stdio

FAILED: unittests/ADT/CMakeFiles/ADTTests.dir/SmallVectorTest.cpp.o 
...
In file included from /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/unittests/ADT/SmallVectorTest.cpp:16:
/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:1526:11: error: comparison of integers of different signs: 'const unsigned long' and 'const int' [-Werror,-Wsign-compare]
  if (lhs == rhs) {
      ~~~ ^  ~~~
/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:1553:12: note: in instantiation of function template specialization 'testing::internal::CmpHelperEQ<unsigned long, int>' requested here
    return CmpHelperEQ(lhs_expression, rhs_expression, lhs, rhs);
           ^
/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/unittests/ADT/SmallVectorTest.cpp:176:3: note: in instantiation of function template specialization 'testing::internal::EqHelper::Compare<unsigned long, int, nullptr>' requested here
  EXPECT_EQ(V.size(), 42);
  ^
/home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/third-party/unittest/googletest/include/gtest/gtest.h:2027:54: note: expanded from macro 'EXPECT_EQ'
  EXPECT_PRED_FORMAT2(::testing::internal::EqHelper::Compare, val1, val2)
                                                     ^
1 error generated.

llvm/include/llvm/ADT/SmallVector.h

 //===- llvm/ADT/SmallVector.h - 'Normally small' vectors --------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 ///

 public:
   SmallVector() : SmallVectorImpl<T>(N) {}
 
   ~SmallVector() {
     // Destroy the constructed elements in the vector.
     this->destroy_range(this->begin(), this->end());
   }
 
-  explicit SmallVector(size_t Size, const T &Value = T())
+  explicit SmallVector(size_t Size)
+    : SmallVectorImpl<T>(N) {
+    this->resize(Size);
+  }
+
+  SmallVector(size_t Size, const T &Value)
     : SmallVectorImpl<T>(N) {
     this->assign(Size, Value);
   }
 
   template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
   SmallVector(ItTy S, ItTy E) : SmallVectorImpl<T>(N) {
     this->append(S, E);
   }

llvm/unittests/ADT/SmallVectorTest.cpp

 //===- llvm/unittest/ADT/SmallVectorTest.cpp ------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
private:
   NonCopyable &operator=(const NonCopyable &) = delete;
 };
 
 LLVM_ATTRIBUTE_USED void CompileTest() {
   SmallVector<NonCopyable, 0> V;
   V.resize(42);
 }
 
+LLVM_ATTRIBUTE_USED void ConstructNonCopyable() {
+  SmallVector<NonCopyable, 0> V(42);
+}
+
 // Assert that v contains the specified values, in order.
 template <typename VectorT>
 void assertValuesInOrder(VectorT &v, size_t size, ...) {
   EXPECT_EQ(size, v.size());
 
   va_list ap;
   va_start(ap, size);
   for (size_t i = 0; i < size; ++i) {