ASTPrinter: add an option to skip keywords with an underscore prefix

[nkcsgexi]

DocSupport will use set this flag to avoid printing __consuming, __owned, and __shared.

rdar://47777848

[nkcsgexi]

@swift-ci please smoke test

[akyrtzi]

I think we should hide them from interface generation in general (disable in PrintOptions::printInterface()). It's similar to how we set SkipPrivateStdlibDecls = true inside that function.

[harlanhaskins]

Looks fine to me. Can you add a test to make sure we don't ever set this to true for parseable interfaces? We always want to print these in parseable interfaces.

[nkcsgexi]

@harlanhaskins what kind of test do you have in mind? We should have existing tests to ensure parseable interface handles underscored attributes, right?

[nkcsgexi]

Maybe i could add an assertion at the entry point of parseable interface generation?

[harlanhaskins]

Alternatively, could you set it explicitly in PrintOptions::printForParseableInterface()?

[nkcsgexi]

@harlanhaskins sure, will do

[nkcsgexi]

@akyrtzi @harlanhaskins Code review comments addressed. Function printKeyword is also updated to handle suffix so that we don't print additional whitespaces when we're not printing a keyword.

[nkcsgexi]

@swift-ci please smoke test

[jrose-apple]

I think we were deliberately printing these. @airspeedswift?

[akyrtzi]

Discussed with @airspeedswift and he is fine with hiding them from documentation but would like them to be present in the generated interface.

[nkcsgexi]

@airspeedswift @akyrtzi The latest commit moved the flag back to documentation generation option so that we won't skip them in the generated interface from IDE.

[nkcsgexi]

@swift-ci please smoke test

[nkcsgexi]

@swift-ci Please smoke test OS X Platform

[nkcsgexi]

@swift-ci please smoke test