[clang][PCH] Incorrect template specialization from precompiled header gets used in source file

When template member function is specialized inside the template class scope and this class definition is in the precompiled header then only first member function template specialization can be used. To reproduce:
```
# build.sh - build script, define LLVM and CLANG_VER variables before use
$LLVM/bin/clang -cc1 -internal-isystem $LLVM/lib/clang/$CLANG_VER/include -nostdsysteminc \
-internal-isystem $LLVM/../clang/test/Headers/Inputs/include -fmodules -x c++-header -emit-pch -o stdafx.pch stdafx.h
$LLVM/bin/clang -cc1 -internal-isystem $LLVM/lib/clang/$CLANG_VER/include -nostdsysteminc -fmodules \
-x c++ -include-pch stdafx.pch main.cpp
```
```
// stdafx.h - precompiled header
#ifndef STDAFX_H
#define STDAFX_H

template
class Test {
public:

template constexpr int get(T t) const;
template <> constexpr int get<0>(T t) const { return t + 10; }
template <> constexpr int get<1>(T t) const { return t + 11; }
};

#endif
```
```
// main.cpp - source file
#include "stdafx.h"

constexpr Test t;
static_assert(t.get<0>(10) == 20);
static_assert(t.get<1>(10) == 21); // This fails

int main(int argc, char **argv) {
return 0;
}
```

Start with the reproduction files stdafx.h and main.cpp, then run the provided clang -cc1 PCH build commands from build.sh. Investigate how the two explicit member-template specializations are recorded and retrieved after PCH loading. Done means both static_assert expressions pass when compiling main.cpp.

Written by the indexing model from the issue text.