[sanitizer] Construct InternalMmapVector without memory allocation.

[ikudrin]

Summary

Construction of InternalMmapVector is often followed by a call to reserve(), which may result in immediate reallocation of the memory for the internal storage. This patch delays that allocation until it is really needed.

[ikudrin]

I'd like to note that this is not a purely theoretical issue. It can be observed, for example, in Symbolizer::ModuleNameOwner on a 64-bit platform with 4K memory pages. The default constructor for storage_ allocates one memory page, which is immediately replaced by a new allocation in the call to reserve() for 1000 elements, which requires 8 * 1000 = 8000 bytes, or two pages.

[eugenis]

LGTM

compiler-rt/lib/sanitizer_common/sanitizer_common.h

 bool operator!=(const InternalMmapVectorNoCtor<T> &lhs,
                 const InternalMmapVectorNoCtor<T> &rhs) {
   return !(lhs == rhs);
 }
 
 template<typename T>
 class InternalMmapVector : public InternalMmapVectorNoCtor<T> {
  public:
-  InternalMmapVector() { InternalMmapVectorNoCtor<T>::Initialize(1); }
+  InternalMmapVector() { InternalMmapVectorNoCtor<T>::Initialize(0); }
   explicit InternalMmapVector(uptr cnt) {
     InternalMmapVectorNoCtor<T>::Initialize(cnt);
     this->resize(cnt);
   }
   ~InternalMmapVector() { InternalMmapVectorNoCtor<T>::Destroy(); }
   // Disallow copies and moves.
   InternalMmapVector(const InternalMmapVector &) = delete;
   InternalMmapVector &operator=(const InternalMmapVector &) = delete;

compiler-rt/lib/sanitizer_common/tests/sanitizer_common_test.cpp

for (uptr i = 0; i < 100; i++) {
     EXPECT_EQ(i, vector[i]);
   }
   for (int i = 99; i >= 0; i--) {
     EXPECT_EQ((uptr)i, vector.back());
     vector.pop_back();
     EXPECT_EQ((uptr)i, vector.size());
   }
   InternalMmapVector<uptr> empty_vector;
-  CHECK_GT(empty_vector.capacity(), 0U);
+  CHECK_EQ(empty_vector.capacity(), 0U);
   CHECK_EQ(0U, empty_vector.size());
 }
 
 TEST(SanitizerCommon, InternalMmapVectorEq) {
   InternalMmapVector<uptr> vector1;
   InternalMmapVector<uptr> vector2;
   for (uptr i = 0; i < 100; i++) {
     vector1.push_back(i);