Support for optimizing and emitting code in LLVM JIT provider.

This commit introduces the ability to actually generate code using
LLVM. In particular, this adds:

- Ability to emit code both in heavily optimized and largely
  unoptimized fashion
- Batching facility to allow functions to be defined in small
  increments, but optimized and emitted in executable form in larger
  batches (for performance and memory efficiency)
- Type and function declaration synchronization between runtime
  generated code and normal postgres code. This is critical to be able
  to access struct fields etc.
- Developer oriented jit_dump_bitcode GUC, for inspecting / debugging
  the generated code.
- per JitContext statistics of number of functions, time spent
  generating code, optimizing, and emitting it.  This will later be
  employed for EXPLAIN support.

This commit doesn't yet contain any code actually generating
functions. That'll follow in later commits.

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

Author: Andres Freund, with contributions by Pierre Ducroquet
Testing-By: Thomas Munro, Peter Eisentraut
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

src/include/jit/llvmjit.h

@@ -30,19 +30,49 @@ extern "C"
 
 
 #include "jit/jit.h"
+#include "nodes/pg_list.h"
 
 
 typedef struct LLVMJitContext
 {
 	JitContext	base;
+
+	/* number of modules created */
+	size_t		module_generation;
+
+	/* current, "open for write", module */
+	LLVMModuleRef module;
+
+	/* is there any pending code that needs to be emitted */
+	bool		compiled;
+
+	/* # of objects emitted, used to generate non-conflicting names */
+	int			counter;
+
+	/* list of handles for code emitted via Orc */
+	List	   *handles;
 } LLVMJitContext;
 
+
+/* type and struct definitions */
+extern LLVMTypeRef TypeSizeT;
+
+extern LLVMValueRef AttributeTemplate;
+extern LLVMValueRef FuncStrlen;
+
+
 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);
+extern LLVMModuleRef llvm_mutable_module(LLVMJitContext *context);
+extern char *llvm_expand_funcname(LLVMJitContext *context, const char *basename);
+extern void *llvm_get_function(LLVMJitContext *context, const char *funcname);
+extern void llvm_split_symbol_name(const char *name, char **modname, char **funcname);
+extern LLVMValueRef llvm_get_decl(LLVMModuleRef mod, LLVMValueRef f);
+extern void llvm_copy_attributes(LLVMValueRef from, LLVMValueRef to);
 
 
 /*