bump clang version required for style check

Merge back changes from stable 1.6

Style check was conducted even if wrong clang-format version
was detected because var was not removed from the cmake cache

string: append, capacity, erase methods

[1.6] Mark allocation_flag constructor as explicit.

Reason: it tests feature introduced with c++20

Call to erase(0) is ambiguous because literal 0 can be treated as integer or null-pointer constant.
Hence equally good candidates in overload resultion are:
- basic_string &erase(size_type index = 0, size_type count = npos)
- iterator erase(const_iterator pos)
We add helper functions to resolve the ambiguity

string: fix size of space needed for null terminator

Fix issue #322

There is a mistake in the unit test itself. The test make wrong
assumption that keys are ordered in the concurrent_hash_map. But
concurrent_hash_map is unordered associative container.

Wrong parameter is passed to constructor. When we create a new
node, the last parameter should be previous head. But in internal_copy()
method the b->tmp_node is passed by mistake.
Assert caught the issue on Windows. The assert is raised only when more
than one elements fit to the same bucket when we do internal_copy().

Having implicit constructor caused unexpected beahviour with
make_persistent_array. For make_persistent version designed to
work with sized arrays calling make_persistent<int[100]>(90) didn't
failed as expected. Instead, '90' was implicitly converted to
allocation_flag.

Merge back changes from stable-1.6

[1.6] Add option to force define IS_TRIVIALLY_COPYABLE

A typo fix.

Merge back changes from stable 1.6

Use libunwind

Change logic for finding font in pmpong example.

Klockwork fixes

Add option to disable hashmap

[1.6] string: fix max_size() return value

string: change sso_capacity so that sizeof(string) is always 32 bytes