Introduce support for 'expand_if_all_available' for flag_groups in CROSSTOOL

With the recent addition of structured variables to CROSSTOOL we now need a way
how to conditionally expand various flag_groups depending on the presence of
particular build variable or its fields. This cl adds this support to flag
groups.

RELNOTES: NONE.
PiperOrigin-RevId: 139466070
1 file changed
tree: 2a3aadaec744933406259976c386d43be4699f48
  1. third_party/