cgo: run CGo parser for all CGo fragments in a file

Previously, libclang was run on each fragment (import "C") separately.
However, in regular Go it's possible for later fragments to refer to
types in earlier fragments so they must have been parsed as one.

This commit changes the behavior to run only one C parser invocation for
each Go file.

cgo/testdata/basic.out.go

@@ -13,3 +13,14 @@ type C.uint32_t = uint32
 type C.uint64_t = uint64
 type C.uint8_t = uint8
 type C.uintptr_t = uintptr
+type C.char uint8
+type C.int int32
+type C.long int32
+type C.longlong int64
+type C.schar int8
+type C.short int16
+type C.uchar uint8
+type C.uint uint32
+type C.ulong uint32
+type C.ulonglong uint64
+type C.ushort uint16