[libcxx/variant] Implement workaround for GCC bug.

A parameter pack is deemed to be uncaptured, which is bogus... but it seems to
be because it's within an expression that involves `decltype` of an uncaptured
pack or something: https://godbolt.org/z/b8z3sh

Drive-by fix for uglified name.

Differential Revision: https://reviews.llvm.org/D86827

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 7d15ece79c16dc3237fc514ff56a69e3d58fbd39
1 file changed