[LangRef] Rework DIExpression docs (#153072)

Factor out most of the DIExpression docs from LangRef.rst into
SourceLevelDebugging.rst

What remains in LangRef is just enough context to make sense of how
DIExpression-as-metadata fits into the IR, including some examples of
the DIExpression syntax.

The rest now lives in the SourceLevelDebugging document, which gives
more context to make sense of DIExpression-as-semantic-entity.

Use sections to clearly separate DWARF opcodes from LLVM internal-only
opcodes, where before the distinction was only explicit in the source
code.

Also make some other changes like fixing typos, using :ref: rather than
unchecked links, and rewording DW_OP_LLVM_entry_value.

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