Back out my heinous hack that tricked the module generation mechanism

into using non-absolute system includes (<foo>)...

... and introduce another hack that is simultaneously more heineous
and more effective. We whitelist Clang-supplied headers that augment
or override system headers (such as float.h, stdarg.h, and
tgmath.h). For these headers, Clang does not provide a module
mapping. Instead, a system-supplied module map can refer to these
headers in a system module, and Clang will look both in its own
include directory and wherever the system-supplied module map
suggests, then adds either or both headers. The end result is that
Clang-supplied headers get merged into the system-supplied module for
the C standard library.

As a drive-by, fix up a few dependencies in the _Builtin_instrinsics
module.

llvm-svn: 149611