Add a command line option to dump function declarations

If -dump-function-declarations is specified, header-abi-dumper outputs
the functions declared but not defined in the input file. The command
line option can be used to dump header files.

Test: header-abi-dumper -suppress-errors -dump-function-declarations system/libfmq/include/fmq/MessageQueue.h -o test.txt --
Bug: 117582158
Bug: 71485848
Change-Id: I85a3944047e085a032779a69b04ac5211f3b9467