Abort if two declaration of the same global have incompatible types

src/kernel_internals/typing/cabs2cil.ml

src/kernel_internals/typing/cabs2cil.mli

tests/syntax/oracle/arg_type.res.oracle

tests/syntax/oracle/composite-tags.res.oracle

tests/syntax/oracle/ghost_parameters.10.res.oracle

tests/syntax/oracle/ghost_parameters.3.res.oracle

tests/syntax/oracle/ghost_parameters.4.res.oracle

tests/syntax/oracle/ghost_parameters.5.res.oracle

tests/syntax/oracle/ghost_parameters.6.res.oracle

tests/syntax/oracle/ghost_parameters.7.res.oracle

tests/syntax/oracle/ghost_parameters.8.res.oracle

tests/syntax/oracle/ghost_parameters.9.res.oracle

tests/syntax/oracle/incompatible_qualifiers.0.res.oracle

tests/syntax/oracle/no_prototype.res.oracle

...yntax/oracle/reject_use_decl_mismatch_bts728.0.res.oracle

...yntax/oracle/reject_use_decl_mismatch_bts728.1.res.oracle

tests/syntax/oracle/rettype.res.oracle

tests/syntax/oracle/syntactic_hook.res.oracle

tests/syntax/syntactic_hook.ml