[lldb] Add support for "external" reports in ThreadSanitizer LLDB plugin

TSan now has the ability to report races on "external" object, i.e. any library class/object that has read-shared write-exclusive threading semantics. The detection and reporting work almost out of the box, but TSan can now provide the type of the object (as a string). This patch implements this into LLDB.

Differential Revision: https://reviews.llvm.org/D30024

llvm-svn: 295342