Define _GNU_SOURCE when importing glibc.

Some functions and types are hidden behind feature flags.
_GNU_SOURCE exposes the most symbols and methods in glibc and musl.

Author: zbowling

lib/ClangImporter/ClangImporter.cpp

@@ -533,6 +533,12 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
       });
     }
   } else {
+    invocationArgStrs.insert(invocationArgStrs.end(), {
+      // Most of the good portable functions used in the Glibc module are hidden
+      // behind this.
+      "-D_GNU_SOURCE",
+    });
+
     // The module map used for Glibc depends on the target we're compiling for,
     // and is not included in the resource directory with the other implicit
     // module maps. It's at {freebsd|linux}/{arch}/glibc.modulemap.