[LLVM-C] Publicly expose getters of MetadataType, TokenType

[LLVM-C] Publicly expose getters of MetadataType, TokenType

Patch by Robert Widmann.

Expose getters for MetadataType and TokenType publicly in the C API.
Discovered a need for these while trying to wrap the intrinsics API.

Differential Revision: https://reviews.llvm.org/D38809

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316762 91177308-0d34-0410-b5e6-96231b3b80d8