Copy remaining libcxx vector tests.
Those test should be ported to test persistent vetctor.
Imported from llvm-mirror/libcxx@5ae92f0

vector ctors

fix coverege travis output issue

travis output has 4MB limitation for an output and recently
it became a limitation for libpmemobj-cpp project

this patch removes llvm-cov output from travis log, but
coverage analysis are still being performed

Functions: begin(), end(), free_data(), size(), capacity()
are being used in ctors/dtor and tests.
This commit provides these functions as well.
Index access operators are added.

cmake: replace unsupported boolean comparison

Imported from llvm-mirror/libcxx@5ae92f0

Modify pmem::obj::array tests

vector: fix _alloc() error handling

We need to cache pmemobj_tx_alloc return value and only after that
assign it to _data, because when pmemobj_tx_alloc fails, it aborts
transaction.

test: fix vector default ctor test

Even if we are removing created files in our test framework,
we should delete manually persistent pointers (for clarity sake).

it was not build previously

check for brace initialization in new expression for specific classes

This line will fail to compile with clang++ <=7.0.0 for C++17
right after destructor will be added to vector class.

Check for issues related to agregate initialization in new expression
for specific classes. Set flag in order to avoid unsafe code in tests.

vector: fix functions description

vector: rm unnecessary cast

Merge back changes from stable-1.5

Change conversion operator from T() to T&() in v<>

persistent_ptr: add unsigned index operator 1.5

Ref: #103

Vector skeleton

Merge back changes from stable-1.5