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/lib/Bitcode/Reader/BitReader.cpp

@@ -25,14 +25,13 @@ using namespace llvm;
    Optionally returns a human-readable error message via OutMessage. */
 LLVMBool LLVMParseBitcode(LLVMMemoryBufferRef MemBuf, LLVMModuleRef *OutModule,
                           char **OutMessage) {
-  return LLVMParseBitcodeInContext(wrap(&getGlobalContext()), MemBuf, OutModule,
+  return LLVMParseBitcodeInContext(LLVMGetGlobalContext(), MemBuf, OutModule,
                                    OutMessage);
 }
 
 LLVMBool LLVMParseBitcode2(LLVMMemoryBufferRef MemBuf,
                            LLVMModuleRef *OutModule) {
-  return LLVMParseBitcodeInContext2(wrap(&getGlobalContext()), MemBuf,
-                                    OutModule);
+  return LLVMParseBitcodeInContext2(LLVMGetGlobalContext(), MemBuf, OutModule);
 }
 
 static void diagnosticHandler(const DiagnosticInfo &DI, void *C) {