[C-API][DIBuilder] Use NameLen in LLVMDIBuilderCreateParameterVariable

Summary: NameLen wasn't being used and caused the parameters in gdb to very long, in my case, crashes in others. Please also perform the correct magical incarnations to have this be applied to the LLVM 7 branch.

Reviewers: whitequark, CodaFi

Reviewed By: CodaFi

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51141

llvm-svn: 340691