VS: Populate `std:c{11,17}` flag table entries for v142

The `-std:c11` option added by commit f7347f28 (MSVC: Record support
for C11 and c_restrict, 2020-08-09, v3.18.2~9^2) needs this flag table
entry to map in the VS IDE properly.

Issue: #21069
19 jobs for vs-c11
Status Job ID Name Coverage
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

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passed ghostflow-stager

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passed ghostflow-stager