[llvm-bcanalyzer] Dump bitcode wrapper header

This patch enables llvm-bcanalyzer to print the bitcode wrapper header
if the file has one, which is needed to test the changes made in
r258627 (bitcode-wrapper-header-armv7m.ll is the test case for r258627).

Differential Revision: http://reviews.llvm.org/D16642

llvm-svn: 259162