Move thread body logic into a private static method

Clang doesn't support adapting calling convention when converting a
non-capturing lambda into a function pointer, and it doesn't
support tagging a lambda with calling convention AFAICT. In
thd_windows.cc, we create the thread body lambda and pass it to
CreateThread(), which fails to build on clang because it cannot
convert the lambda into stdcall function. Note that this bug only
happens when building for x32 architecture. x64 is not affected because
there is only one standard x64 calling convention.

This change fixes this by moving the thread body logic into a private
static method and tagging it with WINAPI (which expands to __stdcall).

src/core/lib/gprpp/thd_windows.cc

@@ -33,10 +33,8 @@
 
 #if defined(_MSC_VER)
 #define thread_local __declspec(thread)
-#define WIN_LAMBDA
 #elif defined(__GNUC__)
 #define thread_local __thread
-#define WIN_LAMBDA WINAPI
 #else
 #error "Unknown compiler - please file a bug report"
 #endif
@@ -71,22 +69,7 @@ class ThreadInternalsWindows
       gpr_free(info_);
       *success = false;
     } else {
-      handle = CreateThread(
-          nullptr, 64 * 1024,
-          [](void* v) WIN_LAMBDA -> DWORD {
-            g_thd_info = static_cast<thd_info*>(v);
-            gpr_mu_lock(&g_thd_info->thread->mu_);
-            while (!g_thd_info->thread->started_) {
-              gpr_cv_wait(&g_thd_info->thread->ready_, &g_thd_info->thread->mu_,
-                          gpr_inf_future(GPR_CLOCK_MONOTONIC));
-            }
-            gpr_mu_unlock(&g_thd_info->thread->mu_);
-            g_thd_info->body(g_thd_info->arg);
-            BOOL ret = SetEvent(g_thd_info->join_event);
-            GPR_ASSERT(ret);
-            return 0;
-          },
-          info_, 0, nullptr);
+      handle = CreateThread(nullptr, 64 * 1024, thread_body, info_, 0, nullptr);
       if (handle == nullptr) {
         destroy_thread();
         *success = false;
@@ -116,6 +99,20 @@ class ThreadInternalsWindows
   }
 
  private:
+  static DWORD WINAPI thread_body(void* v) {
+    g_thd_info = static_cast<thd_info*>(v);
+    gpr_mu_lock(&g_thd_info->thread->mu_);
+    while (!g_thd_info->thread->started_) {
+      gpr_cv_wait(&g_thd_info->thread->ready_, &g_thd_info->thread->mu_,
+                  gpr_inf_future(GPR_CLOCK_MONOTONIC));
+    }
+    gpr_mu_unlock(&g_thd_info->thread->mu_);
+    g_thd_info->body(g_thd_info->arg);
+    BOOL ret = SetEvent(g_thd_info->join_event);
+    GPR_ASSERT(ret);
+    return 0;
+  }
+
   void destroy_thread() {
     CloseHandle(info_->join_event);
     gpr_free(info_);