[clang][deps] Teach dep directive scanner about _Pragma

While we cannot handle `_Pragma` used inside macros, we can handle
this at the top level, and it some projects use the `_Pragma("once")`
spelling like that, which was causing spurious failures in the scanner.

Limitations
* Cannot handle #define ONCE _Pragma("once"), same issue as using
  @import in a macro -- ideally we should diagnose this in obvious cases
* Our LangOpts are currently fixed, so we are not handling u"" strings
  or R"()" strings that require C11/C++11.

rdar://108629982

Differential Revision: https://reviews.llvm.org/D149884