[asan] Set the unhandled exception filter slightly later during startup

VS 2015 moved the priority of their exception filter initializer from
XIY to XCAA. We now set ours to XCAB, which makes it run after both CRT
versions but before user constructors, as it should.

Fixes null_deref.cc and a variety of related tests with VS 2015. Only 4
failures remain.

llvm-svn: 264006

compiler-rt/lib/asan/asan_win.cc

@@ -242,10 +242,16 @@ int __asan_set_seh_filter() {
 }
 
 #if !ASAN_DYNAMIC
-// Put a pointer to __asan_set_seh_filter at the end of the global list
-// of C initializers, after the default EH is set by the CRT.
-#pragma section(".CRT$XIZ", long, read)  // NOLINT
-__declspec(allocate(".CRT$XIZ"))
+// The CRT runs initializers in this order:
+// - C initializers, from XIA to XIZ
+// - C++ initializers, from XCA to XCZ
+// Prior to 2015, the CRT set the unhandled exception filter at priority XIY,
+// near the end of C initialization. Starting in 2015, it was moved to the
+// beginning of C++ initialization. We set our priority to XCAB to run
+// immediately after the CRT runs. This way, our exception filter is called
+// first and we can delegate to their filter if appropriate.
+#pragma section(".CRT$XCAB", long, read)  // NOLINT
+__declspec(allocate(".CRT$XCAB"))
     int (*__intercept_seh)() = __asan_set_seh_filter;
 #endif
 // }}}