[CodeGen] Always use string computed in Sema for PredefinedExpr

We can't use any other string, anyway, because its type wouldn't
match the type of the PredefinedExpr.

With this change, we don't compute a "nice" name for the __func__ global
when it's used in the initializer for a constant. This doesn't seem like
a great loss, and I'm not sure how to fix it without either storing more
information in the AST, or somehow threading through the information
from ExprConstant.cpp.

This could break some situations involving BlockDecl; currently,
CodeGenFunction::EmitPredefinedLValue has some logic to intentionally
emit a string different from what Sema computed.  This code skips that
logic... but that logic can't work correctly in general anyway.  (For
example, sizeof(__func__) returns the wrong result.) Hopefully this
doesn't affect practical code.

Fixes https://bugs.llvm.org/show_bug.cgi?id=40313 .

Differential Revision: https://reviews.llvm.org/D56821

llvm-svn: 351766