Band-aid fix to stop race conditions in llvm errors

This is a big hammer, but should be effective at completely removing a
few issues, including inconsistent error messages and segfaults when
LLVM workers race to report results

LLVM_THREAD_LOCAL has been present in LLVM since 8 months before 3.7
(the earliest supported LLVM version that Rust can use)

Maybe fixes #43402 (third time lucky?)

src/rustllvm/RustWrapper.cpp

@@ -57,7 +57,7 @@ static AtomicOrdering fromRust(LLVMAtomicOrdering Ordering) {
   llvm_unreachable("Invalid LLVMAtomicOrdering value!");
 }
 
-static char *LastError;
+static LLVM_THREAD_LOCAL char *LastError;
 
 extern "C" LLVMMemoryBufferRef
 LLVMRustCreateMemoryBufferWithContentsOfFile(const char *Path) {