[Darwin] Cleanup code via improved GetMacosAlignedVersion()

[yln]

Summary

Checking the OS version via GetMacosAlignedVersion() now works for the
simulator. Let's use it to simplify DyldNeedsEnvVariable().

[delcypher]

LGTM.

compiler-rt/lib/sanitizer_common/sanitizer_mac.h

struct VersionBase {
 
   bool operator==(const VersionType &other) const {
     return major == other.major && minor == other.minor;
   }
   bool operator>=(const VersionType &other) const {
     return major > other.major ||
            (major == other.major && minor >= other.minor);
   }
+  bool operator<(const VersionType &other) const { return !(*this >= other); }
 };
 
 struct MacosVersion : VersionBase<MacosVersion> {
   MacosVersion(u16 major, u16 minor) : VersionBase(major, minor) {}
 };
 
 struct DarwinKernelVersion : VersionBase<DarwinKernelVersion> {
   DarwinKernelVersion(u16 major, u16 minor) : VersionBase(major, minor) {}

compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp

void LeakyResetEnv(const char *name, const char *name_value) {
   }
 }
 
 SANITIZER_WEAK_CXX_DEFAULT_IMPL
 bool ReexecDisabled() {
   return false;
 }
 
-extern "C" SANITIZER_WEAK_ATTRIBUTE double dyldVersionNumber;
-static const double kMinDyldVersionWithAutoInterposition = 360.0;
-
-bool DyldNeedsEnvVariable() {
-  // Although sanitizer support was added to LLVM on OS X 10.7+, GCC users
-  // still may want use them on older systems. On older Darwin platforms, dyld
-  // doesn't export dyldVersionNumber symbol and we simply return true.
-  if (!&dyldVersionNumber) return true;
+static bool DyldNeedsEnvVariable() {
   // If running on OS X 10.11+ or iOS 9.0+, dyld will interpose even if
-  // DYLD_INSERT_LIBRARIES is not set. However, checking OS version via
-  // GetMacosAlignedVersion() doesn't work for the simulator. Let's instead
-  // check `dyldVersionNumber`, which is exported by dyld, against a known
-  // version number from the first OS release where this appeared.
-  return dyldVersionNumber < kMinDyldVersionWithAutoInterposition;
+  // DYLD_INSERT_LIBRARIES is not set.
+  return GetMacosAlignedVersion() < MacosVersion(10, 11);
 }
 
 void MaybeReexec() {
   // FIXME: This should really live in some "InitializePlatform" method.
   MonotonicNanoTime();
 
   if (ReexecDisabled()) return;