* Fixed set_full error when inserting existing item.
* Added test inserting existing value to full set
* Added test inserting existing value to full flat_set and reference_flat_set
* Fixed unordered_set_full error when inserting existing item.
Updated CI scripts
#786 u8string
#783 Cannot insert existing value to full set
#781 etl::multi_span::iterator::operator *() fails if first span is empty
#780 endian enum_type produces useless-cast warnings
#779 hash.h: warnings produced with -Wfloat-equal
* Try to build on Ubuntu-18.04 with Clang-9
* Build with pushes on clang-9 branch too
* Using CC and CXX to set clang-9
* Adding atomic lib for test build on Linux with Clang
* Partial conversion CHECK_EQUAL to CHECK for clang-9
* Replaced CHECK_EQUAL with CHECK
* Replaced CHECK_EQUAL with CHECK
* Replaced CHECK_EQUAL with CHECK
* Replaced CHECK_EQUAL with CHECK
* Replaced CHECK_EQUAL with CHECK
* Replaced CHECK_EQUAL with CHECK
* Replaced CHECK_EQUAL with CHECK
Co-authored-by: finger42 <finger42>
