Add llvm-modextract tool.

This program is for testing features that rely on multi-module bitcode files.
It takes a multi-module bitcode file, extracts one of the modules and writes
it to the output file.

Differential Revision: https://reviews.llvm.org/D26778

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288201 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/BitcodeReader.h

@@ -66,7 +66,9 @@ namespace llvm {
                   bool ShouldLazyLoadMetadata);
 
   public:
-    ArrayRef<uint8_t> getBuffer() const { return Buffer; }
+    StringRef getBuffer() const {
+      return StringRef((const char *)Buffer.begin(), Buffer.size());
+    }
 
     /// Read the bitcode module and prepare for lazy deserialization of function
     /// bodies. If ShouldLazyLoadMetadata is true, lazily load metadata as well.