Allow standards-based attributes to have leading and trailing underscores.

Allow standards-based attributes to have leading and trailing underscores.

This gives library implementers a way to use standards-based attributes that do not conflict with user-defined macros of the same name. Attributes in C2x require this behavior normatively (C2x 6.7.11p4), but there's no reason to not have the same behavior in C++, especially given that such attributes may be used by a C library consumed by a C++ compilation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@369033 91177308-0d34-0410-b5e6-96231b3b80d8