Remove NameLen argument from newly-introduced IR C APIs.

Normally, changing the function signatures of C APIs is disallowed,
but as these two are brand new last week, and haven't been released
yet, it is okay in this instance.

As per discussion in D56556, we will not add NameLen arguments to IR
building APIs, for the following reasons:

1. We do not want to deprecate all of the IR building APIs, just to add a
NameLen argument to each one.

2. Consistency is important, so adding it just to new ones is unfortunate.

3. The IR names are completely optional, useful for readability of IR
only. There is no value in ever supporting nul bytes.

Differential Revision: https://reviews.llvm.org/D56669

llvm-svn: 351076