This change refactors the setting of the compilation mode, so that
instead of using a 'gOfflineCompiler' bool together with a
'gOfflineCompilerOutputType' enum, a single 'gCompilationMode' enum
is used. The default value for gCompilationMode is 'kOnline', which
is equivalent to the previous defaulting of gOfflineCompiler to false.
In addition, it refactors the setting of the compilation cache mode,
so that instead of the 'gForceSpirVCache' and 'gForceSpirVGenerate'
bools, a single 'gCompilationCacheMode' enum is used. The default
value for gCompilationCacheMode is 'kCacheModeCompileIfAbsent', which
is equivalent to the previous defaulting of both booleans to false.
This change also refactors create_openclcpp_program() to avoid saving
and restoring gOfflineCompiler and gOfflineCompilerOutputType. Instead,
it now passes the desired compilation mode as a parameter.
This change removes support for the "source" mode of offline compilation
which is commented with "to be removed in the future as we will use
offline compiler here".
The "source" mode allowed for the CL source to be transformed by
build_script_source.py and then passed to clCreateProgramWithSource(),
and appears to have been developed only for the purpose of testing the
offline compilation infrastructure.
