Switch the default behavior of the Clang<> spelling to opt-in to the C2x...

Switch the default behavior of the Clang<> spelling to opt-in to the C2x attribute spellings. NFC to the attribute spellings themselves.

The Clang<> spelling helper generates a spelling for C++11, GNU, and C2x attribute spellings. Previously, users had to manually opt in to the C2x spelling while we cautiously added attributes to that spelling. Now that majority of attributes are exposed in C2x, we can switch the default.

llvm-svn: 326055