Work around an annoying, non-standard optimization in the glibc

Work around an annoying, non-standard optimization in the glibc
headers, where malloc (and many other libc functions) are declared
with empty throw specifications, e.g.,

  extern void *malloc (__SIZE_TYPE__ __size) throw () __attribute__
  ((__malloc__)) ;

The C++ standard doesn't seem to allow this, and redeclaring malloc as
the standard permits (as follows) resulted in Clang (rightfully!)
complaining about mis-matched exception specifications.

  void *malloc(size_t size);

We work around this by silently propagating an empty throw
specification "throw()" from a function with C linkage declared in a
system header to a redeclaration that has no throw specifier.

Ick.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@95969 91177308-0d34-0410-b5e6-96231b3b80d8