LLVM 11 introduces a hard requirement for SDK 10.12 in order to build for Mac. We want to keep building older LLVMs with 10.11 though, so this patch adds some flexibility so that build-clang can make use of whatever SDK package a particular task pulls from tooltool (but still requesting a deployment target of 10.11).
Differential Revision: https://phabricator.services.mozilla.com/D82621