Clean up linkage handling.

Since the change to the symbol emission strategy, a lot of the
previous attempts to derive the linkage for a symbol from its
AST node has been superfluous. This cleans up all the related
code, as it is really not doing much anymore.

The code to opportunistically internalize symbols is now gone
entirely. It was commented out anyway due to problems with
making it work reliably in the face of the not-so-stable
structure of the AST we get from the frontend (regarding the
modules certain symbols end up in). But in any case internal
linkage woudl also be troublesome for cross-module inlining,
so I gave up on making this work for the time being.

The only (minor) change in the emitted code should be in
.offTi generation, which is untested and disabled by default
anyway (GENERATE_OFFTI).
[cleanup] Remove LDC modifications from mostly unused front-end inliner.

The inliner is also used for default argument handling, so we
need to keep it around, but we no longer need any of the cost
tweaks. In fact, this could even fix some potential bugs with
erroneously accepted default arguments.

We will likely want to bring something like InlineCostVisitor
back for cross-module inlining, but with visitors we can do
this without editing the source now.
[cleanup] getenv_setargv is no longer used in LDC, reduce diff.
[cleanup] Some more alignment of whitespace to DMD.
Merge pull request #737 from klickverbot/random-cleanup

Random cleanup changes