Replace virtual with override.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208324 91177308-0d34-0410-b5e6-96231b3b80d8
Cleanup setFunctionDefinitionAttributes.

Use more specific type, update comments and name style.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208328 91177308-0d34-0410-b5e6-96231b3b80d8
Simplify a few cast<>s.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208331 91177308-0d34-0410-b5e6-96231b3b80d8