Add a new wrapper node for a DILexicalBlock that encapsulates it and a
file. Since it should only be used when necessary propagate it through
the backend code generation and tweak testcases accordingly.

This helps with code like in clang's test/CodeGen/debug-info-line.c where
we have multiple #line directives within a single lexical block and want
to generate only a single block that contains each file change.

Part of rdar://10246360

llvm-svn: 141729