rearranged vhdl tests

Split the VHDL tests up into lexer and parser tests, which are automatically
collected by the CMakeLists.txt file.
removed obsolete dev.cpp test
changed bookkeeping in diagnostics

Diagnostics now keep track of the number of fatal, error, warning, note, and
fixit messages contained within them. The DiagnosticContext implements getters
that check the severity of the diagnostics contained: Fatal errors, errors or
worse, warnings or worse.
added diagnostics type doc

Documented the DiagnosticType numerator and added a doxumentation file for the
root namespace `llhd`.