Improve test coverage of -Wdouble-promotion

This patch adds coverage for additional cases where implicit conversion can
occur (assignment and return).  It also adds tests for some cases where a
warning should occur but none is produced.  These are marked as FIXME.

Differential Revision: http://reviews.llvm.org/D16298


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268075 91177308-0d34-0410-b5e6-96231b3b80d8