Enable LLVM TBAA metadata

[trivial and not reviewed]

Type-Based Alias Analysis metadata emission was disabled
in the LLVM backend because it needed more testing. I've
done the necessary testing:
 - test/users/ferguson has specific alias-analysis test cases
 - I've manually checked the LLVM IR for those tests
 - the Chapel test suite passes everything that should pass
    with --fast --llvm

So this patch enables LLVM TBAA metadata on loads and stores
in the code generator.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21862 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
Tests showing problem passing classes to formal of superclass type




git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21863 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
Fix these tests to reflect correct behaviour Vass pointed out



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21864 3a8e244f-b0f2-452b-bcba-4c88e055c3ca