[ThinLTO] Handle optional args in assembly format for ConstVCalls

Summary:
The AsmWriter was only writing the Args for a ConstVCall if it was
non-empty, however, the LLParser was always expecting it. To aid
in making it optional, surround the ConstVCall VFuncId and Args in
parentheses when writing, then make the Args optional when reading.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49960

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339637 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed