[lldb] Add MSVC STL formatter for std::valarray (#217242)
Pretty-print MSVC STL `std::valarray` from `_Myptr` / `_Mysize`. The
size summary is emitted only when that layout is present so libstdc++
valarrays (same `std::` name, no inline namespace) are not stolen.
References are dereferenced and the element type is taken from `_Myptr`.
Tests: generic valarray suite's MSVC STL category (Windows), covering
empty arrays, references, exact synthetic children, and the libstdc++
dispatch guard.
Part of #24834
Assisted-by: Grok 4.6
Assisted-by: codex-5.6-high
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Co-authored-by: Bjorn Schobben <bjorn.schobben@aimsport.com>
