[sanitizer][asan][win] Only unmap unneeded shadow memory on x86_64

D21942 / 1128db8fe1 added support for
committing shadow memory on demand on Win 64-bit. The reason it is not
enabled on 32-bit wasn't clear but the page table overhead on Windows 7
may be a contributing factor.

In `AsanMapUnmapCallback::OnUnmap`, `FlushUnneededASanShadowMemory` is
called to release shadow memory. It calls `ReleaseMemoryPagesToOS`,
which had been a no-op on Windows, until D95892 /
81b1d3da09 in which it was changed to
unmap full pages that the memory region covers. This was done on both
32-bit and 64-bit.

AddressSanitizerInterface.GetHeapSizeTest appears to fail on i686
targets as a side effect of this. This test allocates and frees a huge
chunk of memory which causes shadow memory to be unmapped immediately.
When the test allocates the chunk of memory a second time, asan tries to
reuse the same shadow memory region, but because the shadow memory has
now been unmapped, it causes an access violation and crashes the test.

x86_64 is not affected, because the code that handles commiting shadow
memory on demand also handles this situation, allowing the test to work
without crashing.

Therefore, this patch changes `FlushUnneededASanShadowMemory` on Windows
to only release/unmap the shadow memory on x86_64 to stop this from
happening on i686.

Differential Revision: https://reviews.llvm.org/D149025

compiler-rt/lib/asan/asan_win.cpp

@@ -194,9 +194,12 @@ void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {
 }
 
 void FlushUnneededASanShadowMemory(uptr p, uptr size) {
+  // Only asan on 64-bit Windows supports committing shadow memory on demand.
+#if SANITIZER_WINDOWS64
   // Since asan's mapping is compacting, the shadow chunk may be
   // not page-aligned, so we only flush the page-aligned portion.
   ReleaseMemoryPagesToOS(MemToShadow(p), MemToShadow(p + size));
+#endif
 }
 
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