[Win64EH] Write .pdata symbol relocations relative to the temporary begin symbol

Previously the relocations pointed at the public user facing,
possibly external symbol.

When the function itself is weak, that symbol may be overridden at
link time, pointing at another strong implementation of the same
function instead. In that case, there's two conflicting pdata entries
pointing at the same address, and the wrong unwind info might end up
used.

Both GCC/binutils and MSVC produce pdata pointing at internal static
symbols. (GCC/binutils point at the .text section just as LLVM does
after this change, MSVC points at special label type symbols with the
type IMAGE_SYM_CLASS_LABEL and names like '$LN4'.)

This fixes unwinding through an overridden "operator new" with a
statically linked C++ library in MinGW mode. (Building libc++ with
-ffunction-sections and linking with --gc-sections might avoid the
issue too.)

This makes the produced object files a little less user friendly
to debug, but with other recent improvements for llvm-readobj, the
unwind info debugging experience should be pretty much the same.

Differential Revision: https://reviews.llvm.org/D109651

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