Bridging headers only import relative paths when creating DSYM

Created by: davidxiasc

When building mixed ObjC/Swift apple binaries that have a bridging header, the bridging header will only check headers by the relative path when generating the DSYM.

This can be illustrated by this example project: https://github.com/davidxiasc/buck-dsym-bridging

If you run buck build :Test#dwarf-with-dsym,iphoneos-arm64

you will get the following error, complaining that Foo.h cannot be found, even though it is under Source/Foo.h. Note that the compiler is able to detect SameDirectory.h as it is located in the same directory as the bridging header.

<path>/Bridging-Header.h:2:9: error: 'Foo.h' file not found
#import "Foo.h"
        ^
<unknown>:0: error: failed to import bridging header 'Bridging-Header.h'

Command failed with exit code 1.

command: [/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc, -frontend, -sdk, /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk, -target, arm64-apple-ios10.0, -import-objc-header, Bridging-Header.h, -Xcc, -Ibuck-out/gen/Test#headers,iph...
<truncated>
...

stderr: <path>/Bridging-Header.h:2:9: error: 'Foo.h' file not found
#import "Foo.h"
        ^
<unknown>:0: error: failed to import bridging header 'Bridging-Header.h'


    When running <swift compile>.
    When building rule //:Test#dwarf-with-dsym,iphoneos-arm64,swift-compile.

However, if instead you run buck :Test#dwarf,iphoneos-arm64, the binary builds correctly.

Oddly, running buck build :Test#dwarf-with-dsym,iphoneos-arm64 after buck :Test#dwarf,iphoneos-arm64 (dirty state) passes, but doesn't actually generate any DSYM.