Added support in Driver which allows -force-single-frontend-invocation and
-emit-objc-header[-path] to be used together in a single invocation.
Added support in tools::Swift to pass -emit-objc-header-path if an Objective-C
header was requested; this is only valid in OutputInfo::Mode::SingleCompile
mode, and an assertion enforces this.
Added a test which ensures that the same header is emitted in with
-force-single-frontend-invocation and without it.
Swift SVN r20185
New rules for the driver (first match):
1. -repl: no input files allowed
2. -parse-sil: one input file allowed
3. -parse-as-library: any number of input files, all treated as Library
4. one input, extension is .sil: treated as SIL
5. one input: treated as Main
6. many inputs: treated as Library by default; "main.swift" is treated as Main
If we want more control here we can also add a -main-file option to explicitly
call out the main source file, but this at least unblocks building an entire
app target (like ListMaker) with a single Swift invocation.
Swift SVN r9890