[unittests] Build LLVMTestingSupport for out-of-source builds

[mgorny]

Summary

The Process/gdb-remote test now requires the LLVMTestingSupport library
that is not installed by LLVM. As a result, when doing an out-of-source
build it fails being unable to find the library. To solve that, build
a local copy of the library when building LLDB with unittests and LLVM
sources available. This is based on how we deal with bundled gtest
sources.

[zturner]

Seems fine to me, beanz?

unittests/CMakeLists.txt

 include(${LLDB_PROJECT_ROOT}/cmake/LLDBDependencies.cmake)
 
 if (LLDB_BUILT_STANDALONE)
   # Build the gtest library needed for unittests, if we have LLVM sources
   # handy.
   if (EXISTS ${LLVM_MAIN_SRC_DIR}/utils/unittest AND NOT TARGET gtest)
     add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/unittest utils/unittest)
   endif()
+  # LLVMTestingSupport library is needed for Process/gdb-remote.
+  if (EXISTS ${LLVM_MAIN_SRC_DIR}/lib/Testing/Support
+      AND NOT TARGET LLVMTestingSupport)
+    add_subdirectory(${LLVM_MAIN_SRC_DIR}/lib/Testing/Support
+      lib/Testing/Support)
+  endif()
 endif()
 
 function(add_lldb_unittest test_name)
   cmake_parse_arguments(ARG
     ""
     ""
     "LINK_LIBS;LINK_COMPONENTS"
     ${ARGN})