Turn local DWARFContext helpers getFileNameForUnit() and getFileLineInfoForCompileUni...

Turn local DWARFContext helpers getFileNameForUnit() and getFileLineInfoForCompileUnit() into full-blowm DWARFDebugLine::LineTable methods.

Summary:
getFileNameForUnit() is basically a wrapper around LineTable::getFileNameByIndex().
Fold its additional functionality (adding the DWARFUnit compilation dir) into
LineTable::getFileNameByIndex().

getFileLineInfoForCompileUnit() is a wrapper around getFileNameForUnit(). As
a function to search the line information by address, it seems natural to put
it in the LineTable also.

Before this commit only the Context with its private helpers could do Linetable
lookups. This newly exposed feature will be used by the DIE dumping code to
get access to file information referenced in DIE attributes.

This commit has already been partly reviewed in D5192 and contained an
additional and a bit controversial 'realpath' call that is left out of this
patch. We can reinstate that realpath code later if it is desirable.

Test Plan:
The patch contains no tests as it should be functionally equivalent to the
previous code. As requested in the last review, I checked if the relative
path handling copied from the Context to LineTable::getFileNameByIndex()
was covered, and indeed the symbolizer tests fail if it is removed.

Reviewers: dblaikie, echristo, aprantl, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5354

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218125 91177308-0d34-0410-b5e6-96231b3b80d8