Cleanup Debug libraries and headers.

Unfortunately, the size of Clang libraries built in Debug mode is bigger than what github currently allows, so cannot commit them
Prototype C++ support
Make Language an enum in ClangAdapter to avoid explosion of language related options
Expand $(ToolchainDir) returned by Atmel Studio when asked for project include paths
Directly look for -D<option> in the toolchain commandline, rather than ask for predefined symbols.

This is necessary for ARM projects, as Atmel Studio implicitly defines the device name macro without adding it to the predefined symbols list
Bump up version to 0.3.4, drop AS 6.0 and add C++