[CMake] Replace the sanitizer support in runtimes build with multilib

[CMake] Replace the sanitizer support in runtimes build with multilib

This is a more generic solution; while the sanitizer support can be used
only for sanitizer instrumented builds, the multilib support can be used
to build other variants such as noexcept which is what we would like to use
in Fuchsia.

The name CMake target name uses the target name, same as for the regular
runtimes build and the name of the multilib, concatenated with '+'. The
libraries are installed in a subdirectory named after the multilib.

Differential Revision: https://reviews.llvm.org/D60926

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@358935 91177308-0d34-0410-b5e6-96231b3b80d8