[llvm] Temporarily disable split stacks

This appears to be causing:
"stack nearly exhausted; compilation time may suffer, and crashes due to stack overflow are likely"
warnings in Swift, so disable it while this issue is investigated.

rdar://154158529
(cherry picked from commit d3a19b5)

Author: Bigcheese

llvm/include/llvm/Support/ProgramStack.h

@@ -12,6 +12,8 @@
 #include "llvm/ADT/STLFunctionalExtras.h"
 #include "llvm/Support/Compiler.h"
 
+// FIXME: Temporarily disable split stacks. rdar://154158529
+#if 0
 // LLVM_HAS_SPLIT_STACKS is exposed in the header because CrashRecoveryContext
 // needs to know if it's running on another thread or not.
 //
@@ -22,6 +24,7 @@
 # define LLVM_HAS_SPLIT_STACKS
 # define LLVM_HAS_SPLIT_STACKS_AARCH64
 #endif
+#endif
 
 namespace llvm {