Remove the UUID::GetAsCString() method which required a buffer to save the 
UUID string in; added UUID::GetAsString() which returns the uuid string in
a std::string.  Updated callers to use the new method.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@181078 91177308-0d34-0410-b5e6-96231b3b80d8