Updated the thread safety attribute definitions to have subjects defined. These are based off what SemaDeclAttr.cpp is checking for in terms of diagnostic reporting. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195274 91177308-0d34-0410-b5e6-96231b3b80d8
Removed a duplicate diagnostic related to attribute subjects for thread safety annotations, and replaced it with the more general attribute diagnostic. Updated the test case in the one instance where wording changed. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195275 91177308-0d34-0410-b5e6-96231b3b80d8