[android][test] Explicitly sign chars to match expectations

`char` is not always signed by default. In fact, the signedness
of the `char` datatype in C is undefined. That means that on some
architectures, the `c_layout.sil` test fails, because it expects
the IR for `char chareth(char a);` to use `signedext`, whereas
some architectures treat these types as unsigned, and thus
`zeroext` is used.

In order to ensure identical signedness on all platforms the tests
run on, explicitly specify the signedness using the compiler flag
`-fsigned-char`.

It would be better to test the signedness for each platform, as
suggested in swiftlang#1103. This is an
interim solution.

Author: modocache

test/IRGen/c_layout.sil

@@ -1,4 +1,6 @@
-// RUN: %target-swift-frontend -I %S/Inputs/abi %s -emit-ir | FileCheck %s --check-prefix=CHECK-%target-cpu
+// FIXME: Rather than passing `-fsigned-char`, this should test each platform's expected char
+//        signedness. For Android, this means implicitly unsigned. See: https://github.com/apple/swift/pull/1103
+// RUN: %target-swift-frontend -Xcc -fsigned-char -I %S/Inputs/abi %s -emit-ir | FileCheck %s --check-prefix=CHECK-%target-cpu
 
 sil_stage canonical
 import c_layout