Sink DwarfDebug::AbstractSPDies down into DwarfFile

This is the first big step to allowing gmlt-like inline scope
information in the skeleton CU. While this commit doesn't change the
functionality, it's only a small step to call
"constructAbstractSubprogramDIE" on both the InfoHolder and the
SkeletonHolder (when in use) and that will at least create the abstract
SP dies in that case, though still not creating the other subprograms.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221051 91177308-0d34-0410-b5e6-96231b3b80d8
[tablegen] Add CustomCallingConv and use it to tablegen-erate the outermost parts of the Mips O32 implementation

Summary:
CustomCallingConv is simply a CallingConv that tablegen should not generate the
implementation for. It allows regular CallingConv's to delegate to these custom
functions. This is (currently) necessary for Mips and we cannot use CCCustom
without having to adapt to the different API that CCCustom uses.

This brings us a bit closer to being able to remove
MipsCC::analyzeCallOperands and MipsCC::analyzeFormalArguments in favour of
the common implementation.

No functional change to the targets.

Depends on D3341

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: vmedic, llvm-commits

Differential Revision: http://reviews.llvm.org/D5965

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221052 91177308-0d34-0410-b5e6-96231b3b80d8
[mips] Removed MipsCC::fixedArgFn(). NFC

Summary:
There is one remaining trace of it in MipsCC::analyzeCallOperands() where
Mips16 might override the calling convention. This will moved into
tablegen-erated code later.

Depends on D5965

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5966

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221053 91177308-0d34-0410-b5e6-96231b3b80d8