```
TLDR: This updates our macOS toolchain to use a newer version of Clang,
cctools (including new dependency on libtapi), LD64 and the macOS SDK.
I've been testing depends builds (HOST=x86_64-apple-darwin16) inside a
Debian Buster Docker container, and running the resultant bitcoind and
bitcoin-qt binaries on a macOS 10.14.4 system. The .dmg generated by a
make deploy also mounts correctly on the same macOS system.
Clang
Upgraded from 3.7.1 to 6.0.1
cctools
cctools 877.8 -> 921
LD64 253.9 -> 409.12
TAPI 1000.10.8
See tpoechtrager/cctools-port and tpoechtrager/apple-libtapi.
macOS SDK
Upgraded from building against the macOS 10.11 SDK to the macOS 10.14
SDK.
```
Backport of core PR16392.
Depends on D5530.