pkgsLLVM.grpc: fix build with clang < 11

Apparently there's an issue where compiling grpc with -std=c++17 fails
unless the clang version is at least 11. Hopefully our default clang
version will be increased to that soon, but until then we need to work
around this problem by setting an older C++ standard.

It's unclear if using C++11 causes further issues, but compiling is
better than not compiling, I suppose.

Contrary to the linked bug report, the darwin stdenv doesn't exhibit
this problem for some reason.

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