llvm-config: Add --has-rtti option

llvm-config: Add --has-rtti option

Summary:
This prints NO if LLVM was built with -fno-rtti or an equivalent flag
and YES otherwise.  The reasons to add -has-rtti rather than adding -fno-rtti
to --cxxflags are:

1. Building LLVM with -fno-rtti does not always mean that client
applications need this flag.

2. Some compilers have a different flag for disabling rtti, and the
compiler being used to build LLVM may not be the compiler being used to
build the application.

Reviewers: echristo, chandlerc, beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11849

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252075 91177308-0d34-0410-b5e6-96231b3b80d8