[asan] Enable -asan-use-private-alias on Darwin/Mach-O, add test for ODR false positi...

[asan] Enable -asan-use-private-alias on Darwin/Mach-O, add test for ODR false positive with LTO (compiler-rt part)

The '-asan-use-private-alias’ option (disabled by default) option is currently only enabled for Linux and ELF, but it also works on Darwin and Mach-O. This option also fixes a known problem with LTO on Darwin (https://github.com/google/sanitizers/issues/647). This patch enables the support for Darwin (but still keeps it off by default) and adds the LTO test case.

Differential Revision: https://reviews.llvm.org/D24292

llvm-svn: 281472