push bytecode decompressor out through APIs. Now the bytecode reader

push bytecode decompressor out through APIs.  Now the bytecode reader
api's look like this:

ModuleProvider *getBytecodeModuleProvider(
  const std::string &Filename,  ///< Name of file to be read
  BCDecompressor_t *BCDC = Compressor::decompressToNewBuffer,
  std::string* ErrMsg = 0,      ///< Optional error message holder
  BytecodeHandler* H = 0        ///< Optional handler for reader events
);

This is ugly, but allows a client to say:

  getBytecodeModuleProvider("foo", 0);

If they do this, there is no dependency on the compression libraries, saving
codesize.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34012 91177308-0d34-0410-b5e6-96231b3b80d8