In order to reliably link to static libraries or individual object
files, we need to take their languages into account as well. For static
libraries this is easy: we just add the static library's list of
compilers to the build target. For extracted objects, we need to only
add the ones for the objects we use.
But we did this really inefficiently -- in fact, downright terribly. We
iterated over all source files from the extracted objects, then tried to
look up a new compiler for them. Even though the extracted objects
already had a list of compilers! This broke once compilers were made
per-subproject, because while the extracted objects have a reference to
all the compilers it needs (just like static archives do, actually) we
might not actually be able to look up that compiler from scratch inside
the current subproject.
Fix this by asking the extracted objects to categorize all its own
sources and return the compilers we want.
Fixes#10579
00f8ced048