Enable asan on our Linux Release pipeline (microsoft#5799)

Address sanitizer can detect all sorts of memory usage related bugs.
Things like overflows, underflows, use-after-free, and (on Linux) leaks.
Due to the great work by @amaiorano, DXC is now asan-clean in our test
suite. Enabling this in our PR builds will allow us to stay that way.

I've chosen the Linux builds because the Linux runtime hooks capture
more errors than on other platforms. It only runs in the Release
configuration because asan can have significant runtime overhead so
running it in a release configuration will mitigate that substantially.
I've also changed the build to use libc++ instead of libstdc++ because
the libc++ headers have additional address sanitizer annotations to
catch misuses of C++ standard library containers.

Author: llvm-beanz

azure-pipelines.yml

@@ -53,7 +53,7 @@ stages:
           configuration: Release
           CC: clang
           CXX: clang++
-          CMAKE_OPTS: -DLLVM_ENABLE_WERROR=On
+          CMAKE_OPTS: -DLLVM_ENABLE_WERROR=On -DLLVM_ENABLE_SANITIZER=Address -DLLVM_ENABLE_LIBCXX=On
           OS: Linux
         Linux_Clang_Debug:
           image: ${{ variables.linux }}