[clang] [Docs] Some updates to the Clang user's manual (PR #151702)

[Aaron Ballman via cfe-commits]

================
@@ -60,29 +60,46 @@ features that depend on what CPU architecture or operating system is
 being compiled for. Please see the :ref:`Target-Specific Features and
 Limitations <target_features>` section for more details.
 
-The rest of the introduction introduces some basic :ref:`compiler
-terminology <terminology>` that is used throughout this manual and
-contains a basic :ref:`introduction to using Clang <basicusage>` as a
-command line compiler.
-
 .. _terminology:
 
 Terminology
 -----------
+* Lexer -- the part of the compiler responsible for converting source code into
+  abstract representations called tokens.
+* Preprocessor -- the part of the compiler responsible for in-place textual
+  replacement of source constructs. When the lexer is required to produce a
+  token, it will run the preprocessor while determining which token to produce.
+  In other words, when the lexer encounters something like `#include` or a macro
+  name, the preprocessor will be used to perform the inclusion or expand the
+  macro name into its replacement list, and return the resulting non-preprocessor
+  token.
+* Parser -- the part of the compiler responsible for determining syntactic
+  correctness of the source code. The parser will request tokens from the lexer
+  and after performing semantic analysis of the production, generates an
+  abstract representation of the source called an AST.
+* Diagnostic -- a message to the user about properties of the source code. For
+  example, errors or warnings and their associated notes.
+* Undefined behavior -- behavior for which the standard imposes no requirements
+  on how the code behaves. Generally speaking, undefined behavior is a bug in
+  the user's code. However, it can also be a place for the compiler to define
+  the behavior, called an extension.
+* Optimizer -- the part of the compiler responsible for transforming user code
+  into faster user code, without changing the semantics of how the code behaves.
+  Note, the optimizer assumes the code has no undefined behavior, so if the code
+  does contain undefined behavior, it will often behave differently depending on
+  which optimization level is enabled.
+* Front end -- the Lexer, Preprocessor, Parser, semantic analysis, and LLVM IR
+  code generation parts of the compiler.
+* Backend -- the parts of the compiler which run after LLVM IR code generation,
----------------
AaronBallman wrote:

Good call on the whitespace, I renamed to Frontend. We do mention middle end in one place, so I'll mention it here.

https://github.com/llvm/llvm-project/pull/151702