internal/importers/objc: fix parsing of newer clang output
internal/importers/objc: fix parsing of newer clang output

Two things broke the clang ast dump parser:

- clang -cc1 didn't parse headers in iPhone mode automatically anymore.
  Add the -triple argument to force it.
- Source positions in the dumps can now contain <scratch space>.

Change-Id: I5d561f781355021f60c94d59e20bf1c1eee76d2a
Reviewed-on: https://go-review.googlesource.com/c/159678
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

golang/mobile,golang/mobile,golang/mobile,golang/mobile