[Support] Add a way to run a function on a detached thread

This roughly mimics `std::thread(...).detach()` except it allows to
customize the stack size. Required for https://reviews.llvm.org/D50993.

I've decided against reusing the existing `llvm_execute_on_thread` because
it's not obvious what to do with the ownership of the passed
function/arguments:

1. If we pass possibly owning functions data to `llvm_execute_on_thread`,
   we'll lose the ability to pass small non-owning non-allocating functions
   for the joining case (as it's used now). Is it important enough?
2. If we use the non-owning interface in the new use case, we'll force
   clients to transfer ownership to the spawned thread manually, but
   similar code would still have to exist inside
   `llvm_execute_on_thread(_async)` anyway (as we can't just pass the same
   non-owning pointer to pthreads and Windows implementations, and would be
   forced to wrap it in some structure, and deal with its ownership.

Patch by Dmitry Kozhevnikov!

Differential Revision: https://reviews.llvm.org/D51103

1 files changed, 0 insertions, 7 deletions

diff --git a/llvm/lib/Support/Windows/Process.inc b/llvm/lib/Support/Windows/Process.inc
index 4b91f9f7fc6..3526e3dee6f 100644
--- a/llvm/lib/Support/Windows/Process.inc
+++ b/llvm/lib/Support/Windows/Process.inc
@@ -439,13 +439,6 @@ const char *Process::ResetColor() {
   return 0;
 }
 
-// Include GetLastError() in a fatal error message.
-static void ReportLastErrorFatal(const char *Msg) {
-  std::string ErrMsg;
-  MakeErrMsg(&ErrMsg, Msg);
-  report_fatal_error(ErrMsg);
-}
-
 unsigned Process::GetRandomNumber() {
   HCRYPTPROV HCPC;
   if (!::CryptAcquireContextW(&HCPC, NULL, NULL, PROV_RSA_FULL,