Both options do not affect the AST content that is serialized into the PCM. This
commit includes the following changes:
1.) Mark `-fvisibility={}` and `-ftype-visibility={}` as benign options.That
means they are no longer considered part of the module hash, which can
reduce the number of module variants.
2.) Add a test to verify the generated LLVM IR is not affected by the default
visibiliy mode in the module.
3.) Add a test to clang-scan-deps to ensure only one module is build, even if
the above mentioned options are used.
This fixes rdar://118246054.
