Cache compilers parsed from config files

- Spack ends up constructing compilers frequently from YAML data.

- This caches the result of parsing the compiler config

- The logic in compilers/__init__.py could use a bigger cleanup, but this
  makes concretization much faster for now.

- on macOS, this also ensures that xcrun is called only twice, as opposed
  to every time a new compiler object is constructed.

lib/spack/spack/compilers/__init__.py

@@ -45,6 +45,9 @@
                         'extra_rpaths']
 _cache_config_file = []
 
+#: cache of compilers constructed from config data, keyed by config entry id.
+_compiler_cache = {}
+
 
 def _auto_compiler_spec(function):
     def converter(cspec_like, *args, **kwargs):
@@ -251,6 +254,10 @@ def compilers_for_arch(arch_spec, scope=None):
 
 
 def compiler_from_config_entry(items):
+    config_id = id(items)
+    compiler = _compiler_cache.get(config_id, None)
+
+    if compiler is None:
         cspec = spack.spec.CompilerSpec(items['spec'])
         os = items.get('operating_system', None)
         target = items.get('target', None)
@@ -278,8 +285,11 @@ def compiler_from_config_entry(items):
         environment = items.get('environment', {})
         extra_rpaths = items.get('extra_rpaths', [])
 
-    return cls(cspec, os, target, compiler_paths, mods, alias,
+        compiler = cls(cspec, os, target, compiler_paths, mods, alias,
                        environment, extra_rpaths, **compiler_flags)
+        _compiler_cache[id(items)] = compiler
+
+    return compiler
 
 
 def get_compilers(config, cspec=None, arch_spec=None):