[lldb] Add linux test for the three-arg version of getProcFile

Also conditionalize the header includes. Not strictly necessary, but
it's weird to include an aix header on non-aix builds, it makes
clang-tidy complain, and breaks build systems which require you to
declare all headers belonging to a library (aka bazel).

Author: labath

lldb/unittests/Host/posix/SupportTest.cpp

@@ -7,10 +7,15 @@
 //===----------------------------------------------------------------------===//
 
 #include "lldb/Host/posix/Support.h"
-#include "lldb/Host/aix/Support.h"
 #include "llvm/Support/Threading.h"
 #include "gtest/gtest.h"
 
+#if defined(_AIX)
+#include "lldb/Host/aix/Support.h"
+#elif defined(__linux__)
+#include "lldb/Host/linux/Support.h"
+#endif
+
 using namespace lldb_private;
 
 #ifndef __APPLE__
@@ -21,9 +26,15 @@ TEST(Support, getProcFile_Pid) {
 }
 #endif // #ifndef __APPLE__
 
-#if defined(_AIX) && defined(LLVM_ENABLE_THREADING)
+#if (defined(_AIX) || defined(__linux__)) && defined(LLVM_ENABLE_THREADING)
 TEST(Support, getProcFile_Tid) {
-  auto BufferOrError = getProcFile(getpid(), llvm::get_threadid(), "lwpstatus");
+  auto BufferOrError = getProcFile(getpid(), llvm::get_threadid(),
+#ifdef _AIX
+                                   "lwpstatus"
+#else
+                                   "status"
+#endif
+  );
   ASSERT_TRUE(BufferOrError);
   ASSERT_TRUE(*BufferOrError);
 }