add code coverage support to Xcode build

This change adds a new Xcode variable, LLDB_ENABLE_COVERAGE.
If set to 1, then the Xcode build will produce a clang
coverage-style build of LLDB.  This can be done with a commandline
invocation such as:

xcodebuild -scheme desktop -configuration Debug build LLDB_ENABLE_COVERAGE=1

Alternatively, the variable can be locally modified from within Xcode
and built with the Xcode IDE.

llvm-svn: 273332