[modules] Use the "redundant #include" diagnostic rather than the "module

[modules] Use the "redundant #include" diagnostic rather than the "module
import can't appear here" diagnostic if an already-visible module is textually
entered (because we have the module map but not the AST file) within a
function/namespace scope.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@288737 91177308-0d34-0410-b5e6-96231b3b80d8