[lldb/test] Use SBPlatform info for lldbplatformutil.getPlatform()

[labath]

Summary

Previously, we just used the platform name. This worked mostly OK, but
it required adding special handling for any unusual (and potentially
downstream) platform plugins, as evidenced by the hardcoding of the
qemu-user platform.

The current implementation was added in
D121605/21c5bb0a636c23ec75b13681c0a6fdb03ecd9c0d, which this essentially
reverts and goes back to the previous method of retrieving the platform
name from the platform triple (the "OS" field).

The motivation for D121605 was the ability to retrieve the process
without constructing an SBDebugger object (which would be necessary in a
world where SBPlatforms are managed by SBDebuggers). However, this world
did not arrive (mainly due to other commitments on my part), and I now
think that if we do want to go in that direction, that we should just
create a dummy/empty SBDebugger object for holding the initial
SBPlatform.

One benefit of D121605 was the unification of getPlatform and
getHostPlatform code paths, and I preserve that benefit by unifying them
in the other direction -- using the host SBPlatform for getHostPlatform.

[JDevlieghere]

LGTM

lldb/packages/Python/lldbsuite/test/dotest.py

 
 def checkDebugServerSupport():
     from lldbsuite.test import lldbplatformutil
     import lldb
 
     skip_msg = "Skipping %s tests, as they are not compatible with remote testing on this platform"
     if lldbplatformutil.platformIsDarwin():
         configuration.skip_categories.append("llgs")
-        if configuration.lldb_platform_name:
+        if lldb.remote_platform:
             # <rdar://problem/34539270>
             configuration.skip_categories.append("debugserver")
             if configuration.verbose:
                 print(skip_msg%"debugserver");
     else:
         configuration.skip_categories.append("debugserver")
-        if configuration.lldb_platform_name and lldbplatformutil.getPlatform() == "windows":
+        if lldb.remote_platform and lldbplatformutil.getPlatform() == "windows":
             configuration.skip_categories.append("llgs")
             if configuration.verbose:
                 print(skip_msg%"lldb-server");
 
 
 def checkForkVForkSupport():
     from lldbsuite.test import lldbplatformutil
 
def run_suite():
     registerFaulthandler()
 
     setupSysPath()
 
     import lldb
     lldb.SBDebugger.Initialize()
     lldb.SBDebugger.PrintStackTraceOnError()
 
-    checkLibcxxSupport()
-    checkLibstdcxxSupport()
-    checkWatchpointSupport()
-    checkDebugInfoSupport()
-    checkDebugServerSupport()
-    checkObjcSupport()
-    checkForkVForkSupport()
-
     # Use host platform by default.
     lldb.remote_platform = None
     lldb.selected_platform = lldb.SBPlatform.GetHostPlatform()
 
     # Now we can also import lldbutil
     from lldbsuite.test import lldbutil
 
     if configuration.lldb_platform_name:
else:
         lldb.remote_platform = None
         configuration.lldb_platform_working_dir = None
         configuration.lldb_platform_url = None
 
     # Set up the working directory.
     # Note that it's not dotest's job to clean this directory.
     lldbutil.mkdir_p(configuration.test_build_dir)
 
+    checkLibcxxSupport()
+    checkLibstdcxxSupport()
+    checkWatchpointSupport()
+    checkDebugInfoSupport()
+    checkDebugServerSupport()
+    checkObjcSupport()
+    checkForkVForkSupport()
+
     skipped_categories_list = ", ".join(configuration.skip_categories)
-    print("Skipping the following test categories: {}".format(skipped_categories_list))
+    print("Skipping the following test categories: {}".format(configuration.skip_categories))
 
     for testdir in configuration.testdirs:
         for (dirpath, dirnames, filenames) in os.walk(testdir):
             visit('Test', dirpath, filenames)
 
     #
     # Now that we have loaded all the test cases, run the whole test suite.
     #

lldb/packages/Python/lldbsuite/test/lldbplatformutil.py

def finalize_build_dictionary(dictionary):
     if target_is_android():
         if dictionary is None:
             dictionary = {}
         dictionary["OS"] = "Android"
         dictionary["PIE"] = 1
     return dictionary
 
 
+def _get_platform_os(p):
+    # Use the triple to determine the platform if set.
+    triple = p.GetTriple()
+    if triple:
+        platform = triple.split('-')[2]
+        if platform.startswith('freebsd'):
+            platform = 'freebsd'
+        elif platform.startswith('netbsd'):
+            platform = 'netbsd'
+        return platform
+
+    return ''
+
+
 def getHostPlatform():
     """Returns the host platform running the test suite."""
-    # Attempts to return a platform name matching a target Triple platform.
-    if sys.platform.startswith('linux'):
-        return 'linux'
-    elif sys.platform.startswith('win32') or sys.platform.startswith('cygwin'):
-        return 'windows'
-    elif sys.platform.startswith('darwin'):
-        return 'macosx'
-    elif sys.platform.startswith('freebsd'):
-        return 'freebsd'
-    elif sys.platform.startswith('netbsd'):
-        return 'netbsd'
-    else:
-        return sys.platform
+    return _get_platform_os(lldb.SBPlatform("host"))
 
 
 def getDarwinOSTriples():
     return lldbplatform.translate(lldbplatform.darwin_all)
 
 def getPlatform():
     """Returns the target platform which the tests are running on."""
     # Use the Apple SDK to determine the platform if set.
     if configuration.apple_sdk:
         platform = configuration.apple_sdk
         dot = platform.find('.')
         if dot != -1:
             platform = platform[:dot]
         if platform == 'iphoneos':
             platform = 'ios'
         return platform
 
-    platform = configuration.lldb_platform_name
-    if platform is None:
-        platform = "host"
-    if platform == "qemu-user":
-        platform = "host"
-    if platform == "host":
-        return getHostPlatform()
-    if platform.startswith("remote-"):
-        return platform[7:]
-    return platform
+    return _get_platform_os(lldb.selected_platform)
 
 
 def platformIsDarwin():
     """Returns true if the OS triple for the selected platform is any valid apple OS"""
     return getPlatform() in getDarwinOSTriples()
 
 
 def findMainThreadCheckerDylib():