Move language conformance tests related to Where clauses to the checkedc repository

[sulekhark]

The tests for _Where clauses need to be segregated into:
a) Tests for language conformance, for example, tests that check that a compiler implements the language extension properly. These should reside in the checkedc repository.
b) Tests that are specific to the compiler implementation. These should reside in the checkedc-clang repository.

Another minor point:
Two Where clause related error messages in the file DiagnosticSemaKinds.td are outside the group of error messages whose CategoryName is Checked C.