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.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316658
91177308-0d34-0410-b5e6-
96231b3b80d8