Put static local variables of inline functions in the function comdat.

The variable (and the GV) is only ever used if the function is. Putting it
in the function's comdat make it easier for the linker to discard them.

The motivating example is

struct S {
  static const int x;
};
// const int S::x = 42;
inline const int *f() {
  static const int y = S::x;
  return &y;
}
const int *g() { return f(); }

With S::x commented out, _ZZ1fvE1y is a variable with a guard variable
that is initialized by f.

With S::x present, _ZZ1fvE1y is a constant.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224369 91177308-0d34-0410-b5e6-96231b3b80d8