builder, cgo: support function definitions in CGo headers

For example, the following did not work before but does work with this
change:

    // int add(int a, int b) {
    //   return a + b;
    // }
    import "C"

    func main() {
        println("add:", C.add(3, 5))
    }

Even better, the functions in the header are compiled together with the
rest of the Go code and so they can be optimized together! Currently,
inlining is not yet allowed but const-propagation across functions
works. This should be improved in the future.

loader/loader.go

@@ -72,6 +72,7 @@ type Package struct {
 	Files      []*ast.File
 	FileHashes map[string][]byte
 	CFlags     []string // CFlags used during CGo preprocessing (only set if CGo is used)
+	CGoHeaders []string // text above 'import "C"' lines
 	Pkg        *types.Package
 	info       types.Info
 }
@@ -397,8 +398,9 @@ func (p *Package) parseFiles() ([]*ast.File, error) {
 		if p.program.clangHeaders != "" {
 			initialCFlags = append(initialCFlags, "-Xclang", "-internal-isystem", "-Xclang", p.program.clangHeaders)
 		}
-		generated, cflags, ldflags, accessedFiles, errs := cgo.Process(files, p.program.workingDir, p.program.fset, initialCFlags)
+		generated, headerCode, cflags, ldflags, accessedFiles, errs := cgo.Process(files, p.program.workingDir, p.program.fset, initialCFlags)
 		p.CFlags = append(initialCFlags, cflags...)
+		p.CGoHeaders = headerCode
 		for path, hash := range accessedFiles {
 			p.FileHashes[path] = hash
 		}