[asan] Turn -fsanitize-address-use-after-scope on by default [clang part]

AddressSanitizer has an optional compile-time flag, -fsanitize-address-use-after-scope, which enables detection of use-after-scope bugs. We'd like to have this feature on by default, because it is already very well tested, it's used in several projects already (LLVM automatically enables it when using -DLLVM_USE_SANITIZER=Address), it's low overhead and there are no known issues or incompatibilities.

This patch enables use-after-scope by default via the Clang driver, where we set true as the default value for AsanUseAfterScope. This also causes the lifetime markers to be generated whenever fsanitize=address is used. This has some nice consequences, e.g. we now have line numbers for all local variables.

Differential Revision: https://reviews.llvm.org/D31479

llvm-svn: 299174