This allows code that only deals with BodyContent (post-decoding) to
still be able to report on missing items within the associated body while
providing a suitable source location.
Currently only deals with the literal HCL structure. Later we will also
support HIL parsing and evaluation within strings, to achieve parity with
existing uses of HCL/HIL together. For now, this has parity with uses of
HCL alone, with the exception that float and int values are not
distinguished, because cty does not make this distinction.
