[modules] Make GNUMode a normal language option to fix module compilation.

GNUMode shouldn't be a benign language option because it influences the
resulting AST when checking for the existence of GNUMode-specific macro
"linux" (e.g. by having code inside #ifdef linux). This patch marks it as a
normal language option so it gets correctly passed to the compiler invocation
for the used modules.

The added test case illustrated this because it compiles without modules, but
fails when using modules.

Patch by Raphael Isemann (D30496)!

llvm-svn: 297030