[DebugInfo] Support DWARF v5 source code embedding extension

In DWARF v5 the Line Number Program Header is extensible, allowing values with
new content types. This vendor extension to DWARF v5 allows source text to be
embedded directly in the line tables of the debug line section.

Add new flag (-g[no-]embed-source) to Driver and CC1 which indicates
that source should be passed through to LLVM during CodeGen.

Differential Revision: https://reviews.llvm.org/D42766

llvm-svn: 326102