Add optional DICompileUnit to DIBuilder + make outliner debug info use it

Previously, the DIBuilder didn't expose functionality to set its compile unit
in any other way than calling createCompileUnit. This meant that the outliner,
which creates new functions, had to create a new compile unit for its debug
info.

This commit adds an optional parameter in the DIBuilder's constructor which
lets you set its CU at construction.

It also changes the MachineOutliner so that it keeps track of the DISubprograms
for each outlined sequence. If debugging information is requested, then it
uses one of the outlined sequence's DISubprograms to grab a CU. It then uses
that CU to construct the DISubprogram for the new outlined function.

The test has also been updated to reflect this change.

See https://reviews.llvm.org/D42254 for more information. Also see the e-mail
discussion on D42254 in llvm-commits for more context.

llvm-svn: 322992