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

llvm-svn: 307357