[cmake] Add support for using the standalone leaks sanitizer with LLVM.
[cmake] Add support for using the standalone leaks sanitizer with LLVM.

This commit causes LLVM_USE_SANITIZER to now accept the "Leaks" option. This
will cause cmake to pass in -fsanitize=leak in all of the appropriate places.

I am making this change so that I can setup a linux bot that only detects
leaks.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@305839 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 864e349715f5e32049bef560a23777b465f6c8ca)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm