-time-ir-parsing flag

This breaks the layering of the Support library. We can't add an
implementation side to IRReader because it refers directly to entities
only accessible as part of the IR, AsmParser, and BitcodeReader
libraries. It can only be used in a context where all of those libraries
will be available.

We'll need to find some other way to get this functionality, and
hopefully solve the long-standing layering problem of IRReader.h...

llvm-svn: 177695