stdlib.h. There were numerous problems with forward declaring 'malloc' and
'free', but the most important is that these are reserved by POSIX and may be
implemented via a function-like macro.

As suggested by Dale Johannesen, I'm instead guarding the only include of this
in our builtin headers with __STDC_HOSTED__, and I've removed the include of
the header from the test suite. I'll discuss with folks whether we want to have
a hosted section of the test suite or not, and add it (and perhaps other tests)
back there if that's the direction.

llvm-svn: 119958