Bitcode: Introduce BitcodeWriter interface.

This interface allows clients to write multiple modules to a single
bitcode file. Also introduce the llvm-cat utility which can be used
to create a bitcode file containing multiple modules.

Differential Revision: https://reviews.llvm.org/D26179

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288195 91177308-0d34-0410-b5e6-96231b3b80d8

test/Bitcode/Inputs/multi-module.ll

@@ -0,0 +1,3 @@
+define void @f2() {
+  ret void
+}