ggml : add coverage measurement for Clang, increase test coverage, F16 ggml_sum ...

ggml : add coverage measurement for Clang, increase test coverage, F16 ggml_sum (#377)

* First shot at adding clang/llvm coverage analysis

* Fix for compiler dependency

* Reducing dimensions in test-opt

* cmake : try to fix test coverage build + CI

* cmake : fix CMAKE option + CI

* Adding some tests for half precision floating point tests

* Adding missing tests for unary operations

* Some more tests for unary operations

* Fix syntax error.

* Fix bug in relu derivative computation

* Revert testing change

* ggml : style fixes

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Co-authored-by: Georgi Gerganov <redacted>