Basic JIT provider and error handling infrastructure.

This commit introduces:

1) JIT provider abstraction, which allows JIT functionality to be
   implemented in separate shared libraries. That's desirable because
   it allows to install JIT support as a separate package, and because
   it allows experimentation with different forms of JITing.
2) JITContexts which can be, using functions introduced in follow up
   commits, used to emit JITed functions, and have them be cleaned up
   on error.
3) The outline of a LLVM JIT provider, which will be fleshed out in
   subsequent commits.

Documentation for GUCs added, and for JIT in general, will be added in
later commits.

Author: Andres Freund, with architectural input from Jeff Davis
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

src/include/jit/llvmjit.h

@@ -0,0 +1,51 @@
+/*-------------------------------------------------------------------------
+ * llvmjit.h
+ *	  LLVM JIT provider.
+ *
+ * Copyright (c) 2016-2018, PostgreSQL Global Development Group
+ *
+ * src/include/jit/llvmjit.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef LLVMJIT_H
+#define LLVMJIT_H
+
+#ifndef USE_LLVM
+#error "llvmjit.h should only be included by code dealing with llvm"
+#endif
+
+#include <llvm-c/Types.h>
+
+
+/*
+ * File needs to be includable by both C and C++ code, and include other
+ * headers doing the same. Therefore wrap C portion in our own extern "C" if
+ * in C++ mode.
+ */
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+
+#include "jit/jit.h"
+
+
+typedef struct LLVMJitContext
+{
+	JitContext	base;
+} LLVMJitContext;
+
+extern void llvm_enter_fatal_on_oom(void);
+extern void llvm_leave_fatal_on_oom(void);
+extern void llvm_reset_after_error(void);
+extern void llvm_assert_in_fatal_section(void);
+
+extern LLVMJitContext *llvm_create_context(int jitFlags);
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif							/* LLVMJIT_H */