Commits on Feb 21, 2024

1. [Serialization] Record whether the ODR is skipped (llvm#82302) …

   Close llvm#80570.
   
   In
   
   llvm@a0b6747,
   we skipped ODR checks for decls in GMF. Then it should be natural to
   skip storing the ODR values in BMI.
   
   Generally it should be fine as long as the writer and the reader keep
   consistent.
   
   However, the use of preamble in clangd shows the tricky part.
   
   For,
   
   ```
   // test.cpp
   module;
   
   // any one off these is enough to crash clangd
   // #include <iostream>
   // #include <string_view>
   // #include <cmath>
   // #include <system_error>
   // #include <new>
   // #include <bit>
   // probably many more
   
   // only ok with libc++, not the system provided libstdc++ 13.2.1
   
   // these are ok
   
   export module test;
   ```
   
   clangd will store the headers as preamble to speedup the parsing and the
   preamble reuses the serialization techniques. (Generally we'd call the
   preamble as PCH. However it is not true strictly. I've tested the PCH
   wouldn't be problematic.) However, the tricky part is that the preamble
   is not modules. It literally serialiaze and deserialize things. So
   before clangd parsing the above test module, clangd will serialize the
   headers into the preamble. Note that there is no concept like GMF now.
   So the ODR bits are stored. However, when clangd parse the file
   actually, the decls from preamble are thought as in GMF literally, then
   hte ODR bits are skipped. Then mismatch happens.
   
   To solve the problem, this patch adds another bit for decls to record
   whether or not the ODR bits are skipped.
   
   (cherry picked from commit 49775b1)

   

   ChuanqiXu9 authored and tstellar committed Feb 21, 2024
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