Reland r230446, "MS ABI: Try to respect external AST source record layouts"

It broke test/PCH/headersearch.cpp because it was using -Wpadding, which
only works for Itanium layout. Before this commit, we would use Itanium
record layout when using PCH, which is crazy. Now that the test uses an
explicit Itanium triple, we can reland.

llvm-svn: 230525