Check module signature when the module has already been loaded

We may need to verify the signature on subsequent imports as well, just
like we verify the size/modtime:
@import A;
@import B; // imports A
@import C; // imports A

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221569 91177308-0d34-0410-b5e6-96231b3b80d8