Split out the IRReader header and the utility functions it provides into
its own library. These functions are bridging between the bitcode reader
and the ll parser which are in different libraries. Previously we didn't
have any good library to do this, and instead played fast and loose with
a "header only" set of interfaces in the Support library. This really
doesn't work well as evidenced by the recent attempt to add timing logic
to the these routines.

As part of this, make them normal functions rather than weird inline
functions, and sink the implementation into the library. Also clean up
the header to be nice and minimal.

This requires updating lots of build system dependencies to specify that
the IRReader library is needed, and several source files to not
implicitly rely upon the header file to transitively include all manner
of other headers.

If you are using IRReader.h, this commit will break you (the header
moved) and you'll need to also update your library usage to include
'irreader'. I will commit the corresponding change to Clang momentarily.
The IRReader header is now part of its own library. Update the include
line and the library dependencies to reflect this.