[clang][Interp] Create only globals when initializing a global variable

For this code:

struct O {
  int &&j;
};

O o1(0);

The generated AST for the initializer of o1 is:

VarDecl 0x62100006ab08 <array.cpp:119:3, col:9> col:5 o1 'O':'O' parenlistinit
`-ExprWithCleanups 0x62100006b250 <col:7, col:9> 'O':'O'
  `-CXXParenListInitExpr 0x62100006b210 <col:7, col:9> 'O':'O'
    `-MaterializeTemporaryExpr 0x62100006b1f0 <col:8> 'int' xvalue
      `-IntegerLiteral 0x62100006abd0 <col:8> 'int' 0

Before this patch, we create a local temporary variable for the
MaterializeTemporaryExpr and destroy it again when destroying the
EvalEmitter we create to interpret the initializer. However, since
O::j is a reference, this reference now points to a local variable
that doesn't exist anymore.

Differential Revision: https://reviews.llvm.org/D156453