[LSan] Fix a crash when LSan hits a guard page while scanning thread stack for pointers.

Summary:
In some cases stack pointer register (SP) doesn't point into the thread
stack: e.g. if one is using swapcontext(). In this case LSan
conservatively tries to scan the whole thread stack for pointers.
However, thread stack (at least in glibc implementation) may also
include guard pages, causing LSan to crash when it's reading from them.

One of the solutions is to use a pthread_attr_getguardsize() to adjust
the calculated stack boundaries. However, here we're just using
IsAccessibleMemoryRange to skip guard pages and make the code (slightly)
less platform-specific.

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17116

llvm-svn: 260554