[ASan] Enable -asan-stack-dynamic-alloca by default.

[ASan] Enable -asan-stack-dynamic-alloca by default.

By default, store all local variables in dynamic alloca instead of
static one. It reduces the stack space usage in use-after-return mode
(dynamic alloca will not be called if the local variables are stored
in a fake stack), and improves the debug info quality for local
variables (they will not be described relatively to %rbp/%rsp, which
are assumed to be clobbered by function calls).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228336 91177308-0d34-0410-b5e6-96231b3b80d8