Remove Platform references from the Host module

Summary:
These were used by Host::LaunchProcess to "resolve" the executable it
was about to launch. The only parts of Platform::ResolveExecutable, which
seem to be relevant here are the FileSpec::ResolvePath and
ResolveExecutableLocation calls.

The rest (most) of that function deals with selecting an architecture
out of a fat binary and making sure we are able to create a Module with that
slice. These are reasonable actions when selecting a binary to debug,
but not for a generic process launching framework (it's technically even
wrong because we should be able to launch a binary with execute
permissions only, but trying to parse such file will obviously fail).

I remove the platform call by inlining the relevant FileSpec calls and
ignoring the rest of the Platform::ResolveExecutable code.  The
architecture found by the slice-searching code is being ignored already
anyway, as we use the one specified in the LaunchInfo, so the only
effect of this should be a different error message in case the
executable does not contain the requested architecture -- before we
would get an error message from the Platform class, but now we will get
an error from the actual posix_spawn syscall (this is only relevant on
mac, as it's the only target supporting fat binaries).

Launching targets for debugging should not be affected as here the
executable is pre-resolved at the point when the Target is created.

Reviewers: jingham, clayborg

Subscribers: lldb-commits, emaste

Differential Revision: https://reviews.llvm.org/D41902

llvm-svn: 322935