[asan] Implement delayed activation of AddressSanitizer

This change adds ASAN_OPTIONS=start_deactivated=1 flag. When present, ASan will
start in "deactivated" mode, with no heap poisoning, no quarantine, no stack
trace gathering, and minimal redzones. All this features come back when
__asan_init is called for the constructor of an instrumented library.

The primary use case for this feature is Android. Code itself is not
Android-specific, and this patch includes a Linux test for it.

llvm-svn: 199377