ARROW-8608: [C++] Update vendored 'variant.hpp' to fix CUDA 10.2
ARROW-8608: [C++] Update vendored 'variant.hpp' to fix CUDA 10.2

Updates vendored `variant.hpp` to [latest](https://github.com/mpark/variant/commit/d1cdfdd3f2ed80710ba4d671fe6bffaa3e28201a) with fixes for CUDA 10.2 nvcc.

Fixes https://issues.apache.org/jira/browse/ARROW-8608.

Closes #7053 from trxcllnt/ARROW-8608

Lead-authored-by: Paul Taylor <9b5e68c334274b572dadf4941c134a2f08be9921@me.com>
Co-authored-by: ptaylor <9b5e68c334274b572dadf4941c134a2f08be9921@me.com>
Signed-off-by: François Saint-Jacques <76801b7d5fe2c448a8401d2246372be49d372480@gmail.com>

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