In response to some discussions on IRC, tweak the wording of the new

In response to some discussions on IRC, tweak the wording of the new
-Wsometimes-uninitialized diagnostics to make it clearer that the cause
of the issue may be a condition which must always evaluate to true or
false, rather than an uninitialized variable.

To emphasize this, add a new note with a fixit which removes the
impossible condition or replaces it with a constant.

Also, downgrade the diagnostic from -Wsometimes-uninitialized to
-Wconditional-uninitialized when it applies to a range-based for loop,
since the condition is not written explicitly in the code in that case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157511 91177308-0d34-0410-b5e6-96231b3b80d8