This reverts commit 81fc3add1e627c23b7270fe2739cdacc09063e54.

This breaks some LLDB tests, e.g.
SymbolFile/DWARF/x86/no_unique_address-with-bitfields.cpp:

lldb: ../llvm-project/clang/lib/AST/Decl.cpp:4604: unsigned int clang::FieldDecl::getBitWidthValue() const: Assertion `isa<ConstantExpr>(getBitWidth())' failed.

Save the bitwidth value as a `ConstantExpr` with the value set. Remove
the `ASTContext` parameter from `getBitWidthValue()`, so the latter
simply returns the value from the `ConstantExpr` instead of
constant-evaluating the bitwidth expression every time it is called.

... elements.

Unfortunately, that means we can't use the __builtin_bit_cast
implementation for this.

See the discussion in

https://github.com/llvm/llvm-project/pull/119678#issuecomment-2539680746
and
https://github.com/llvm/llvm-project/pull/119544#issuecomment-2539678561

In the case of primitive arrays, we can do this once and reuse the value
for all elements.

Like we do in ExprConstant.cpp.

In certain cases (i.e. long double on x86), the bit with we get from the
floating point semantics is different than the type size we compute for
the BitCast instruction. Pass this along to DoBitCast, so in there we
can check only the relevant bits for being initialized.

This also fixes a weirdness we still had in DoBitCast.