Harmonize GNU- and C++-style attribute spellings.

Most attributes will now use the Clang<"name"> construct to provide both __attribute__((name)) and [[clang::name]] syntaxes for the attribute. Attributes deviating from this should be marked with a comment explaining why they are not supported under both spellings. Common reasons are: the attribute is provided by some other specification that controls the syntax or the attribute cannot be exposed under a particular spelling for some given reason.

Because this is a mechanical change that only introduces new spellings, there are no test cases for the commit.

llvm-svn: 320752