[llvm-c] Correctly check for existence of native AsmParser, AsmPrinter, Disassembler

Also, properly name the functions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194141 91177308-0d34-0410-b5e6-96231b3b80d8
[llvm-c] Specify explicit namespace in LLVMLoadLibraryPermanently

Presence of using namespace llvm depended on several #ifdef's, and
this broke the build on mswin32.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194142 91177308-0d34-0410-b5e6-96231b3b80d8