Support embedding natvis files in PDBs.

Natvis is a debug language supported by Visual Studio for
specifying custom visualizers.  The /NATVIS option is an
undocumented link.exe flag which will take a .natvis file
and "inject" it into the PDB.  This way, you can ship the
debug visualizers for a program along with the PDB, which
is very useful for postmortem debugging.

This is implemented by adding a new "named stream" to the
PDB with a special name of /src/files/<natvis file name>
and simply copying the contents of the xml into this file.

Additionally, we need to emit a single stream named
/src/headerblock which contains a hash table of embedded
files to records describing them.

This patch adds this functionality, including the /NATVIS
option to lld-link.

Differential Revision: https://reviews.llvm.org/D44328

llvm-svn: 327895