Intrinsics: add LLVMHalfElementsVectorType constraint

This is like the LLVMMatchType, except the verifier checks that the
second argument is a vector with the same base type and half the
number of elements.

This will be used by the ARM64 backend.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205079 91177308-0d34-0410-b5e6-96231b3b80d8
Make GetCPISymbol a virtual method.

ARM64 for iOS is going to want to emit these symbols in a
linker-private style for efficiency, but other targets probably don't
want that behaviour.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205080 91177308-0d34-0410-b5e6-96231b3b80d8
MachO: allow each section to have a linker-private symbol

The upcoming ARM64 backend doesn't have section-relative relocations,
so we give each section its own symbol to provide this functionality.
Of course, it doesn't need to appear in the final executable, so
linker-private is the best kind for this purpose.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205081 91177308-0d34-0410-b5e6-96231b3b80d8
MachO: actually set linker-private prefix at MC level.

This was accidentally omitted from r205081.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205083 91177308-0d34-0410-b5e6-96231b3b80d8
MachO: Add linker-optimisation hint framework to MC.

Another part of the ARM64 backend (so tests will be following soon).
This is currently used by the linker to relax adrp/ldr pairs into nops
where possible, though could well be more broadly applicable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205084 91177308-0d34-0410-b5e6-96231b3b80d8