Added a new preprocessor macro: __has_declspec_attribute. This can be used as a way to determine whether Clang supports a __declspec spelling for a given attribute, similar to  __has_attribute and __has_cpp_attribute.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223467 91177308-0d34-0410-b5e6-96231b3b80d8