cmath: Support clang's -fdelayed-template-parsing

r283051 added some functions to cmath (in namespace std) that have the
same name as functions in math.h (in the global namespace).  Clang's
limited support for `-fdelayed-template-parsing` chokes on this.  Rename
the ones in `cmath` and their uses in `complex` and the test.

rdar://problem/32848355

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@307357 91177308-0d34-0410-b5e6-96231b3b80d8

utils/libcxx/test/config.py

@@ -440,6 +440,9 @@ class Configuration(object):
             # C++17 aligned allocation.
             self.config.available_features.add('no-aligned-allocation')
 
+        if self.cxx.hasCompileFlag('-fdelayed-template-parsing'):
+            self.config.available_features.add('fdelayed-template-parsing')
+
         if self.get_lit_bool('has_libatomic', False):
             self.config.available_features.add('libatomic')