IRToASTVisitor: preserve native calling conventions in declarations and calls #347

Rellic v0.2.3 creates function declarations using a default Clang
FunctionProtoType::ExtProtoInfo. It does not copy Function::getCallingConv().
Calls through function pointers similarly construct a default C function type
without reading CallInst::getCallingConv().

An LLVM module with x86_stdcallcc, x86_fastcallcc, and x86_thiscallcc
therefore becomes plain C declarations, definitions, and indirect calls. This
changes argument registers and stack cleanup when the result is compiled for
i686-pc-windows-msvc.

A synthetic module contains eight functions: four sum functions with different
conventions, one caller, and three callers through pointers. For example, these
are declarations and a call from the synthetic module:

define x86_stdcallcc i32 @stdcall_sum(i32 %a, i32 %b, i32 %c) {
  %ab = add i32 %a, %b
  %abc = add i32 %ab, %c
  ret i32 %abc
}

define x86_fastcallcc i32 @fastcall_sum(i32 inreg %a, i32 inreg %b, i32 %c) {
  %ab = add i32 %a, %b
  %abc = add i32 %ab, %c
  ret i32 %abc
}

define x86_thiscallcc i32 @thiscall_sum(ptr %receiver, i32 %a, i32 %b) {
  %value = load i32, ptr %receiver
  %ab = add i32 %a, %b
  %abc = add i32 %ab, %value
  ret i32 %abc
}

define i32 @call_indirect_stdcall(ptr %fn, i32 %a, i32 %b, i32 %c) {
  %result = call x86_stdcallcc i32 %fn(i32 %a, i32 %b, i32 %c)
  ret i32 %result
}

The generated declarations are currently ordinary C declarations, and the
pointer call has an ordinary C pointer cast. A static roundtrip can be checked
without executing argument cases:

rellic-decomp --input calling-conventions.ll --output calling-conventions.c
clang --target=i686-pc-windows-msvc -S -emit-llvm -O0 \
  calling-conventions.c -o roundtrip.ll

The return to LLVM drops the three named native conventions. A local fix
preserves both Clang function-type conventions and printed declaration
attributes, and creates convention-bearing typedefs for indirect calls. The
typedefs avoid invalid placement of attributes inside a C cast type-name.
Unsupported LLVM-only conventions produce an explicit error.

Static comparison of the complete synthetic module linked as two DLLs gives
byte-exact code for the four individual sum functions and all three indirect
callers. The combined caller has an instruction-ordering difference after
optimization; that difference is retained in the report.

No third-party binary, machine-code sample, runtime inputs, local identifiers,
or product-specific information is included in this report.