Allow weak/selectany external definitions in header units. (#162713)
weak and selectany are mechanisms for allowing the linker to resolve ODR
violations. [module.import/6] states
> A header unit shall not contain a definition of a non-inline function
or variable whose name has external linkage.
But this prevents compiling any headers with such weak symbols defined.
These occur in eg. some Windows SDK headers like `DirectXMath.h`.
```
#ifndef XMGLOBALCONST
#if defined(__GNUC__) && !defined(__MINGW32__)
#define XMGLOBALCONST extern const __attribute__((weak))
#else
#define XMGLOBALCONST extern const __declspec(selectany)
#endif
#endif
XMGLOBALCONST XMVECTORF32 g_XMSinCoefficients0 = { { { -0.16666667f, +0.0083333310f, -0.00019840874f, +2.7525562e-06f } } };
```
Proposed solution: Do not emit `diag::err_extern_def_in_header_unit` if
the `FD` or `VDecl` have either `SelectAnyAttr` or `WeakAttr`.
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