Merge GetCompileUnitAtOffset + GetCompileUnitContainingDIEOffset

These two methods are very similar and various refactorizations need to modify
both similar ways.

One could also just remove GetCompileUnitAtOffset and make
GetCompileUnitContainingDIEOffset to also accept offset of the CU itself
(currently it accepts only DIE offsets after the CU header).
But that would be less safe regarding some internal sanity checking.

Further code refactorization has been suggested by Pavel Labath.

Differential Revision: https://reviews.llvm.org/D61498

llvm-svn: 360038