[lldb][test] Disable procfile by thread ID test when LLVM_ENABLE_THREADS is not defined

When LLVM_ENABLE_THREADS is not defined, llvm::get_threadid returns 0 which
makes this test case fail.

This is a pretty niche setting, Linaro uses it to stop lld crashing our 32 bit
containers. So the test will get plenty of runs elsewhere.

In lldb's code it's not getting the current thread ID anyway, it's using
a value it got from ptrace. So even if that copy of lldb was built with
LLVM_ENABLE_THREADS off, it should still be able to debug threads.

Author: DavidSpickett

lldb/unittests/Host/linux/SupportTest.cpp

@@ -18,8 +18,10 @@ TEST(Support, getProcFile_Pid) {
   ASSERT_TRUE(*BufferOrError);
 }
 
+#ifdef LLVM_ENABLE_THREADING
 TEST(Support, getProcFile_Tid) {
   auto BufferOrError = getProcFile(getpid(), llvm::get_threadid(), "comm");
   ASSERT_TRUE(BufferOrError);
   ASSERT_TRUE(*BufferOrError);
 }
+#endif /*ifdef LLVM_ENABLE_THREADING */