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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320752 91177308-0d34-0410-b5e6-96231b3b80d8