runtime: fix bitmap marking to exclude ASAN redzone

[qinlonglong123]

When ASAN is enabled, a redzone is appended to allocation size by the compiler-rt.
This causes dataSize to be larger than the actual user data size, which affects
bitmap calculation when the redzone size is comparable to or larger than typ.Size_.

To fix this, use actualSize() to subtract the redzone size before computing the
bitmap mask and scanSize, ensuring only the actual user data region is reflected
in the bitmap.

Fixes #80136

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from 秦龙:

Patch Set 1: Code-Review+1

(2 comments)

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