Update link strategy for sanitizer runtime libraries on Linux:

1. Always put static sanitizer runtimes to the front of the linker
invocation line. This was already done for all sanitizers except UBSan:
in case user provides static libstdc++ we need to make sure that new/delete
operator definitions are picked from sanitizer runtimes instead of libstdc++.
We have to put UBSan runtime first for similar reasons: it depends on some
libstdc++ parts (e.g. __dynamic_cast function), and has to go first in
link line to ensure these functions will be picked up from libstdc++.

2. Put sanitizer libraries system dependencies (-ldl, -lpthread etc.) right
after sanitizer runtimes. This will ensure these libraries participate in
the link even if user provided -Wl,-as-needed flag. This should fix PR15823.

3. In case we link in several sanitizer runtimes (e.g. "ubsan", "ubsan_cxx"
and "san"), add system dependencies (-ldl, -lpthread, ...) only once.

llvm-svn: 215940