[LPM] Clean up the use of TLS in pretty stack trace and disable it

entirely when threads are not enabled. This should allow anyone who
needs to bootstrap or cope with a host loader without TLS support to
limp along without threading support.

There is still some bug in the PPC TLS stuff that is not worked around.
I'm getting access to a machine to reproduce and debug this further.
There is some chance that I'll have to add a terrible workaround for
PPC.

There is also some problem with iOS, but I have no ability to really
evaluate what the issue is there. I'm leaving it to folks maintaining
that platform to suggest a path forward -- personally I don't see any
useful path forward that supports threading in LLVM but does so without
support for *very basic* TLS. Note that we don't need more than some
pointers, and we don't need constructors, destructors, or any of the
other fanciness which remains widely unimplemented.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227411 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/PrettyStackTrace.cpp

@@ -16,6 +16,7 @@
 #include "llvm-c/Core.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Config/config.h"     // Get autoconf configuration settings
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/Watchdog.h"
 #include "llvm/Support/raw_ostream.h"
@@ -29,26 +30,7 @@ using namespace llvm;
 // We need a thread local pointer to manage the stack of our stack trace
 // objects, but we *really* cannot tolerate destructors running and do not want
 // to pay any overhead of synchronizing. As a consequence, we use a raw
-// thread-local variable. Some day, we should be able to use a limited subset
-// of C++11's thread_local, but compilers aren't up for it today.
-// FIXME: This should be moved to a Compiler.h abstraction.
-#ifdef _MSC_VER
-// MSVC supports this with a __declspec.
-#define LLVM_THREAD_LOCAL __declspec(thread)
-#else
-// Clang, GCC, and all compatible compilers tend to use __thread. But we need
-// to work aronud a bug in the combination of Clang's compilation of
-// local-dynamic TLS and the ppc64 linker relocations which we do by forcing to
-// global-dynamic (called in most documents "general dynamic").
-// FIXME: Make this conditional on the Clang version once this is fixed in
-// top-of-tree.
-#if defined(__clang__) && defined(__powerpc64__)
-#define LLVM_THREAD_LOCAL __thread __attribute__((tls_model("global-dynamic")))
-#else
-#define LLVM_THREAD_LOCAL __thread
-#endif
-#endif
-
+// thread-local variable.
 static LLVM_THREAD_LOCAL const PrettyStackTraceEntry *PrettyStackTraceHead =
     nullptr;