Proposal: allow setting cflags with @cImport

[ehaas]

Some clang code-generation options can affect the behavior of translate-c by changing the sizes of types. Two notable examples are -fshort-enums (common in embedded systems) and -fshort-wchar (for building programs to run under WINE). With #8664 these flags can be passed to zig translate-c using -cflags but there is currently no way to pass them when using @cImport.

I propose adding a builtin @cFlags(flags: []const []const u8) void. Like cDefine/cInclude/etc it would only be valid within a cImport block. This function would append flags to the array of flags to be passed to clang for the current cImport instance.

These flags will also need to be included in the cache manifest since they can change the output translated code.

[andrewrk]

Will be solved instead by #20630.