DebugInfo: Drop confusing forwarding API from DILexicalBlockFile

Remove `DILexicalBlockFile::getScope()` (whose last use was removed from
clang in r234245), which illegally returned a `DILexicalBlock` despite
its scope sometimes being an `MDSubprogram`.  Also remove the
`getLineNumber()` and `getColumnNumber()` methods that just forwarded to
`DILexicalBlock`'s versions, since there don't seem to be any callers.

Note that the block of code removed from `DebugInfo.cpp` was actually
dead code, since `isLexicalBlock()` (the previous branch) always returns
true when `isLexicalBlockFile()` returns true.

An earlier (broken and untested) version of this was squashed into
r234222 and reverted in r234225.

llvm-svn: 234246