Check module signature when the module has already been loaded

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