Create lex_phases.md

Author: shafik

lex_phases.md

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+- if a splice results in a character sequence that matches the syntax of a universal-character-name, the behavior is undefined. *\[lex.phases\]/p2*
+    - Example from [Stacoverflow question](https://stackoverflow.com/q/43824729/1708801):
+
+    ```cpp
+    const char* p = "\\
+    u0041";
+    ```
+
+    - Rationale
+        - [DR 787](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#787)
+        - [N3881 “Fixing the specification of universal-character-names”](http://open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3881.pdf)
+
+- If a character sequence that matches the syntax of a universal-character-name is produced by token concatenation (19.3.3), the behavior is undefined. *\[lex.phases\]/p4*
+    - example:  
+
+    ```cpp
+    #define GUARD_NAME ï ## _GUARD // UB per current spec
+    #define COLUMN "ï" ## _column // UB per current spec
+    ```
+    
+    - Rationale
+        - [DR 787](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#787)
+        - [N3881 “Fixing the specification of universal-character-names”](http://open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3881.pdf)
+