Allow writing calling convention attributes on function types.

Allow writing calling convention attributes on function types.

Calling convention attributes notionally appertain to the function type -- they modify the mangling of the function, change the behavior of assignment operations, etc. This commit allows the calling convention attributes to be written in the type position as well as the declaration position.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331459 91177308-0d34-0410-b5e6-96231b3b80d8