Add new intrinsic LDC_never_inline.

LDC_never_inline is a complementary intrinsic to LDC_allow_inline.
It tells the LLVM optimizer to never inline a function. This can be
useful if inlining creates incorrect code.
A possible application is core.thread.getStackTop().
Fix comments
Merge pull request #310 from redstar/noinline

Add new intrinsic LDC_no_inline.