Add a `%darwin_min_target_with_tls_support` lit substitution.

Summary:
This substitution expands to the appropriate minimum deployment target
flag where thread local storage (TLS) was first introduced on Darwin
platforms. For all other platforms the substitution expands to an empty
string.

E.g. for macOS the substitution expands to `-mmacosx-version-min=10.12`

This patch adds support for the substitution (and future substitutions)
by doing a minor refactor and then uses the substitution in the relevant
TSan tests.

rdar://problem/59568956

Reviewers: yln, kubamracek, dvyukov, vitalybuka

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D74802