76b2915fdb
Leverage perf-training flow for BOLT profile collection, enabling reproducible
BOLT optimization. Remove the use of bootstrapped build for profile collection.
Test Plan:
- Regular (single-stage) build
```
$ cmake ... -C .../clang/cmake/caches/BOLT.cmake
$ ninja clang-bolt
...
[21/24] Instrumenting clang binary with BOLT
[21/24] Generating BOLT profile for Clang
[23/24] Merging BOLT fdata
Profile from 2 files merged.
[24/24] Optimizing Clang with BOLT
...
1291202496 : executed instructions (-1.1%)
27005133 : taken branches (-71.5%)
...
```
- Two stage build (ThinLTO+InstPGO)
```
$ cmake ... -C .../clang/cmake/caches/BOLT.cmake -C .../clang/cmake/caches/BOLT-PGO.cmake
$ ninja clang-bolt
$ ninja stage2-clang-bolt
...
[2756/2759] Instrumenting clang binary with BOLT
[2756/2759] Generating BOLT profile for Clang
[2758/2759] Merging BOLT fdata
[2759/2759] Optimizing Clang with BOLT
...
BOLT-INFO: 7092 out of 184104 functions in the binary (3.9%) have non-empty execution profile
756531927 : executed instructions (-0.5%)
15399400 : taken branches (-40.3%)
...
```
Reviewed By: beanz
Differential Revision: https://reviews.llvm.org/D143553
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