[clang][dataflow] Add `Environment::initializeFieldsWithValues()`. (#81239)

This function will be useful when we change the behavior of record-type
prvalues
so that they directly initialize the associated result object. See also
the
comment here for more details:


https://github.com/llvm/llvm-project/blob/9e73656af524a2c592978aec91de67316c5ce69f/clang/include/clang/Analysis/FlowSensitive/DataflowEnvironment.h#L354

As part of this patch, we document and assert that synthetic fields may
not have
reference type.

There is no practical use case for this: A `StorageLocation` may not
have
reference type, and a synthetic field of the corresponding non-reference
type
can serve the same purpose.