pull .ll and .bc writing out of the ASTConsumer destructors into some top

level code in clang.  This is a cleanup, but does implement "-o" for 
-emit-llvm.  One effect of this is that "clang foo.c -emit-llvm" will now
emit into foo.ll instead of stdout.  Use "clang foo.c -emit-llvm -o -" or 
"clang < foo.c -emit-llvm" to get the old behavior.

llvm-svn: 46791