Add a new attribute definition spelling, Clang<"attr">, that expands to two...

Add a new attribute definition spelling, Clang<"attr">, that expands to two attribute spellings: GNU<"attr"> and CXX11<"clang", "attr">. This is similar to how the GCC spelling works and is intended to be used for attributes introduced for Clang.

Changes all existing attributes that currently use GNU<"attr"> and CXX11<"clang", "attr> spellings to instead use the Clang<"attr"> spelling.

No additional tests are necessary because the existing tests already use both spellings for the attributes converted to the new spelling. No functional changes are expected.

llvm-svn: 316658