[libc++] Set the target triple by default in the standalone build

[ldionne]

Summary

Even though the standalone build is deprecated, some people are still
relying on it (including libc++ itself for some configurations). Setting
the target triple will ensure that the build and the test suite behaves
consistently in the standalone and normal builds.

libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake

elseif (EXISTS "${LLVM_MAIN_SRC_DIR}/cmake/modules")
   list(APPEND CMAKE_MODULE_PATH "${LLVM_MAIN_SRC_DIR}/cmake/modules")
 else()
   message(FATAL_ERROR "Neither ${LLVM_CMAKE_PATH} nor ${LLVM_MAIN_SRC_DIR}/cmake/modules found. "
                       "This is not a supported configuration.")
 endif()
 
 message(STATUS "Configuring for standalone build.")
 
+# By default, we target the host, but this can be overridden at CMake invocation time.
+include(GetHostTriple)
+get_host_triple(LLVM_INFERRED_HOST_TRIPLE)
+set(LLVM_HOST_TRIPLE "${LLVM_INFERRED_HOST_TRIPLE}" CACHE STRING "Host on which LLVM binaries will run")
+set(LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_HOST_TRIPLE}" CACHE STRING "Target triple used by default.")
+
 # Add LLVM Functions --------------------------------------------------------
 if (WIN32)
   set(LLVM_ON_UNIX 0)
   set(LLVM_ON_WIN32 1)
 else()
   set(LLVM_ON_UNIX 1)
   set(LLVM_ON_WIN32 0)
 endif()

libcxx/utils/libcxx/test/dsl.py

def featureTestMacros(config, flags=''):
   Return a dictionary of feature test macros.
 
   The keys are strings representing feature test macros, and the values are
   integers representing the value of the macro.
   """
   allMacros = compilerMacros(config, flags)
   return {m: int(v.rstrip('LlUu')) for (m, v) in allMacros.items() if m.startswith('__cpp_')}
 
-@_memoizeExpensiveOperation(lambda c: (c.substitutions, c.environment))
-def getHostTriple(config):
-  """
-  Returns the default triple of the compiler.
-
-  TODO: This shouldn't be necessary here - ideally the user would always pass
-        the triple as a parameter. This is done to support the legacy standalone
-        builds, which don't set the triple.
-  """
-  with _makeConfigTest(config) as test:
-    unparsedOutput, err, exitCode, timeoutInfo = _executeScriptInternal(test, [
-      "%{cxx} %{flags} %{compile_flags} -dumpmachine"
-    ])
-    output = re.search(r"# command output:\n(.+)\n", unparsedOutput).group(1)
-    return output
-
 def _appendToSubstitution(substitutions, key, value):
   return [(k, v + ' ' + value) if k == key else (k, v) for (k, v) in substitutions]
 
 def _prependToSubstitution(substitutions, key, value):
   return [(k, value + ' ' + v) if k == key else (k, v) for (k, v) in substitutions]
 
 
 class ConfigAction(object):

libcxx/utils/libcxx/test/params.py

def getStdFlag(cfg, std):
   }
   if hasCompileFlag(cfg, '-std='+std):
     return '-std='+std
   if std in fallbacks and hasCompileFlag(cfg, '-std='+fallbacks[std]):
     return '-std='+fallbacks[std]
   return None
 
 DEFAULT_PARAMETERS = [
-  Parameter(name='target_triple', type=str, default=getHostTriple,
+  Parameter(name='target_triple', type=str,
             help="The target triple to compile the test suite for. This must be "
                  "compatible with the target that the tests will be run on.",
             actions=lambda triple: filter(None, [
               AddFeature('target={}'.format(triple)),
               AddFlagIfSupported('--target={}'.format(triple)),
             ])),
 
   Parameter(name='std', choices=_allStandards, type=str,