Add four new functions and one new enum to the C API:

LLVMGetThreadLocalMode - exposes GlobalVariable::getThreadLocalMode
LLVMSetThreadLocalMode - exposes GlobalVariable::setThreadLocalMode
LLVMIsExternallyInitialized - exposes GlobalVariable::isExternallyInitialized
LLVMSetExternallyInitialized - exposes GlobalVariable::setExternallyInitialized
LLVMThreadLocalMode - maps to GlobalVariable::ThreadLocalMode

Patch by Moritz Maxeiner!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179588 91177308-0d34-0410-b5e6-96231b3b80d8