SBCommandReturnObject: change LLDB_RECORD_METHOD(..., FILE *, ...) to use LLDB_RECORD_DUMMY

POSIX says FILE is a typedef to a structure containing information about
a file. The structure is unspecified, i.e. it may be an incomplete type, as is the case on musl
(`struct _IO_FILE` is an implementation detail that is not exposed).

`LLDB_RECORD_METHOD(..., (FILE *), ...)` transitively uses sizeof(FILE)
and requires the structure to be complete.  Change it to
LLDB_RECORD_DUMMY to fix the build failure on musl (regression of
D57475).

Reviewed By: JDevlieghere, labath, lawrence_danna

Differential Revision: https://reviews.llvm.org/D68872

llvm-svn: 375072