Add const. NFC.

This adds const to a few methods that already return const references or
creates a const version when they reterun non-const references.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221666 91177308-0d34-0410-b5e6-96231b3b80d8
MCAsmParserExtension has a copy of the MCAsmParser. Use it.

Base classes were storing a second copy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221667 91177308-0d34-0410-b5e6-96231b3b80d8
[NVPTX] Remove dead code in NVPTXTargetTransformInfo (NFC)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221668 91177308-0d34-0410-b5e6-96231b3b80d8
Only run the gold plugin tests if gold supports the targets we test with.

This fixes pr21345.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221669 91177308-0d34-0410-b5e6-96231b3b80d8