clang: explicitly enable/disable RTTI support with the compiler

Add -frtti to the compiler with the base flags case so that RTTI is
enabled by default with clang. Add its inverse analog,
cxx_no_rtti_flags in order to test the case with RTTI off, similar to
gcc.

This reduces the amount of testing/support overhead needed in the
non-RTTI case with clang, as the tests currently fail when these two
features are off with version 1.8.1. This something I used in when
investigating test failures on FreeBSD, as the tests that rely on
RTTI were failing with googletest 1.8.1 on the OS platform.

More investigation is being done to determine how this should be fixed
on FreeBSD 11.2-RELEASE with ports, as the package doesn't currently
compile the tests, and when enabled (based on my WIP diff), the tests
fail in similar ways.

Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>