Remove every uses of getGlobalContext() in LLVM (but the C API)

At the same time, fixes InstructionsTest::CastInst unittest: yes
you can leave the IR in an invalid state and exit when you don't
destroy the context (like the global one), no longer now.

This is the first part of http://reviews.llvm.org/D19094

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266379

llvm/tools/llvm-as/llvm-as.cpp

@@ -91,7 +91,7 @@ int main(int argc, char **argv) {
   // Print a stack trace if we signal out.
   sys::PrintStackTraceOnErrorSignal();
   PrettyStackTraceProgram X(argc, argv);
-  LLVMContext &Context = getGlobalContext();
+  LLVMContext Context;
   llvm_shutdown_obj Y; // Call llvm_shutdown() on exit.
   cl::ParseCommandLineOptions(argc, argv, "llvm .ll -> .bc assembler\n");