[sanitizer_common] Use posix_spawn when starting a subprocess

[nspark]

Summary

When available, enables use of posix_spawn when starting a subprocess

Passes local testing (test/check-llvm-instrumentation) on Ubuntu 20.04

Fixes: https://github.com/google/sanitizers/issues/979

See also: https://reviews.llvm.org/D48451

[nspark]

Fix lint errors with clang-format on sanitizer_posix_libcdep.cpp

[nspark]

Squashed last two commits, since the earlier patch (pre clang-format) was not appearing in Phabricator.

compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp

 // run-time libraries and implements libc-dependent POSIX-specific functions
 // from sanitizer_libc.h.
 //===----------------------------------------------------------------------===//
 
 #include "sanitizer_platform.h"
 
 #if SANITIZER_POSIX
 
-#include "sanitizer_common.h"
-#include "sanitizer_flags.h"
-#include "sanitizer_platform_limits_netbsd.h"
-#include "sanitizer_platform_limits_posix.h"
-#include "sanitizer_platform_limits_solaris.h"
-#include "sanitizer_posix.h"
-#include "sanitizer_procmaps.h"
-
 #include <errno.h>
 #include <fcntl.h>
 #include <pthread.h>
 #include <signal.h>
 #include <spawn.h>
 #include <stdlib.h>
 #include <sys/mman.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
 
+#include "sanitizer_common.h"
+#include "sanitizer_flags.h"
+#include "sanitizer_platform_limits_netbsd.h"
+#include "sanitizer_platform_limits_posix.h"
+#include "sanitizer_platform_limits_solaris.h"
+#include "sanitizer_posix.h"
+#include "sanitizer_procmaps.h"
+
 #if SANITIZER_FREEBSD
 // The MAP_NORESERVE define has been removed in FreeBSD 11.x, and even before
 // that, it was never implemented.  So just define it to zero.
 #undef MAP_NORESERVE
 #define MAP_NORESERVE 0
 #endif
 
 typedef void (*sa_sigaction_t)(int, siginfo_t *, void *);