Make DWARFContext more thread safe.

llvm-gsymutil uses a DWARFContext from multiple threads as it parses each compile unit. As it finds issues it might end up dumping a DIE to an output stream which can cause accesses to the DWARFContext from multiple threads. In llvm-gsymutil it can end up dumping a DIE from multiple threads into thread specific stream which was causing DWARFContext::getTUIndex() to be called and can crash the process.

This fix puts a recursive mutex into the DWARFContext class and makes most APIs threadsafe for access. Many of the methods in DWARFContext will check if a member variable has been filled in yet, and parse what is needed and populate a member variagle with the results. Now a mutex protects these functions.

Differential Revision: https://reviews.llvm.org/D157459