[llvm-c] Add LLVMPrintModuleToString.

Like LLVMDumpModule but returns the string (that needs to be freed
with LLVMDisposeMessage) instead of printing it to stderr.

Differential Revision: http://llvm-reviews.chandlerc.com/D1941



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192821 91177308-0d34-0410-b5e6-96231b3b80d8