Check default target triple during config

[urnathan]

Summary

I wasted some time setting a default target triple to 'arm-eabi', thinking it would be canonicalized. The build succeeded but testing blew up because utils/lit/lit/llvm/config.py has:

def make_itanium_abi_triple(self, triple):
     m = re.match(r'(\w+)-(\w+)-(\w+)', triple)
    ...

and that regex expects at least two '-'. Notice that regex isn't anchored and some triplets have '.' (solaris2.10 is the example I recall, dunno if anyone cares enough now though, it also doesn't affect the semantics of what's being checked there either.)

Anyway, this inserts a regex check in set_llvm_target_triplet to look for something at least reasonable at cmake config time -- somewhat earlier.

Includes a bit of line wrap reformatting while there.

llvm/cmake/modules/SetTargetTriple.cmake

 macro(set_llvm_target_triple)
-  set(LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE_default}" CACHE STRING
+  set(LLVM_DEFAULT_TARGET_TRIPLE
+    "${LLVM_DEFAULT_TARGET_TRIPLE_default}" CACHE STRING
     "Default target for which LLVM will generate code." )
+  if (NOT LLVM_DEFAULT_TARGET_TRIPLE
+      MATCHES "^[0-9A-Za-z_]+-[0-9A-Za-z_]+-[0-9A-Za-z_.]+(-[0-9A-Za-z_]+)?\$")
+    message(FATAL_ERROR
+      "LLVM_DEFAULT_TARGET_TRIPLE '${LLVM_DEFAULT_TARGET_TRIPLE}' is malformed")
+  endif ()
   if (TARGET_TRIPLE)
-    message(WARNING "TARGET_TRIPLE is deprecated and will be removed in a future release. "
+    message(WARNING
+      "TARGET_TRIPLE is deprecated and will be removed in a future release. "
       "Please use LLVM_DEFAULT_TARGET_TRIPLE instead.")
     set(LLVM_TARGET_TRIPLE "${TARGET_TRIPLE}")
   else()
     set(LLVM_TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE}")
   endif()
   message(STATUS "LLVM host triple: ${LLVM_HOST_TRIPLE}")
   message(STATUS "LLVM default target triple: ${LLVM_DEFAULT_TARGET_TRIPLE}")
 endmacro()