[llvm] MCFragment: Use trailing data for fixed-size part (PR #150846)

[David Spickett via llvm-commits]

DavidSpickett wrote:

The new test is failing on Linaro's 32-bit builders:
```
******************** TEST 'LLVM :: MC/ELF/many-instructions.s' FAILED ********************
Exit Code: 1
Command Output (stderr):
--
/home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/llvm-mc -filetype=obj -triple=x86_64 /home/tcwg-buildbot/worker/clang-armv7-global-isel/llvm/llvm/test/MC/ELF/many-instructions.s -o /dev/null -debug-only=mc-dump 2>&1 | grep -E -o '[0-9]+ Data Size:[0-9]+' | /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/FileCheck /home/tcwg-buildbot/worker/clang-armv7-global-isel/llvm/llvm/test/MC/ELF/many-instructions.s # RUN: at line 2
+ /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/llvm-mc -filetype=obj -triple=x86_64 /home/tcwg-buildbot/worker/clang-armv7-global-isel/llvm/llvm/test/MC/ELF/many-instructions.s -o /dev/null -debug-only=mc-dump
+ grep -E -o '[0-9]+ Data Size:[0-9]+'
+ /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/FileCheck /home/tcwg-buildbot/worker/clang-armv7-global-isel/llvm/llvm/test/MC/ELF/many-instructions.s
/home/tcwg-buildbot/worker/clang-armv7-global-isel/llvm/llvm/test/MC/ELF/many-instructions.s:5:10: error: CHECK: expected string not found in input
# CHECK: 0 Data Size:16220
         ^
<stdin>:1:1: note: scanning from here
0 Data Size:16240
^
Input file: <stdin>
Check file: /home/tcwg-buildbot/worker/clang-armv7-global-isel/llvm/llvm/test/MC/ELF/many-instructions.s
-dump-input=help explains the following input dump.
Input was:
<<<<<<
         1: 0 Data Size:16240 
check:5     X~~~~~~~~~~~~~~~~~ error: no match found
         2: 16240 Data Size:140 
check:5     ~~~~~~~~~~~~~~~~~~~~
>>>>>>
--
```
https://lab.llvm.org/buildbot/#/builders/39/builds/7363 is one example.

I think these stats are the size of structures inside of MC, which would mean they could change size if the ABI is different? (we only have 32-bit Arm builds, not 32-bit x86 so idk if it's a problem there)

https://github.com/llvm/llvm-project/pull/150846