Find CoreCLR sources via find_package rather than requiring -DWITH_CORECLR

[JosephTremoulet]

CMake has a standard pattern for locating dependencies. Our current requirement that -DWITH_CORECLR be specified on the command line causes some friction in the LLVM build, where none of the other cmake options are required (see D12679). When the CoreCLR supports using find_package to locate these files (see dotnet/coreclr#1521), we should update our cmake files to use that, so that -DWITH_CORECLR is optional and need only be specified when you want to override the default location.