[KeyInstr] Enable Key Instructions support in LLVM

Set LLVM_EXPERIMENTAL_KEY_INSTRUCTIONS=ON by default. This enables support for
Key Instructions in LLVM by default, it does not enable the feature by default.

This does have an affect on compile time, which looks to have mostly been
"paid for" (if that argument stands) by my PR llvm#143399.

From compile-time-tracker:

1. this patch
2. PR llvm#143399
3. base
```
    Commit  stage1-O3  stage1-ReleaseThinLTO  stage1-ReleaseLTO-g  stage1-O0-g  stage1-aarch64-O3  stage1-aarch64-O0-g  stage2-O3      stage2-O0-g      stage2-clang
        1.  61213M (+0.02%)  77397M (+0.01%)  89413M (+0.08%)  18865M (+0.13%)  68670M (+0.01%)  23100M (+0.12%)  53409M (+0.00%)  16550M (+0.20%)  34060797M (+0.02%)
	2.  61201M (-0.01%)  77389M (+0.00%)  89343M (-0.06%)  18841M (-0.16%)  68666M (+0.00%)  23073M (-0.16%)  53408M (-0.01%)  16518M (+0.01%)  34054978M (+0.00%)
	3.  61207M (+0.00%)  77386M (-0.01%)  89396M (-0.02%)  18871M (-0.02%)  68665M (+0.01%)  23109M (+0.02%)  53415M (-0.02%)  16516M (-0.00%)  34054300M (-0.00%)
```

Compare 2-1: https://llvm-compile-time-tracker.com/compare.php?from=89490929c34f45842df1588cf78d836f51c2c222&to=7d00712c28bbcc8a8e00d672f2f7c109fb5823c9&stat=instructions%3Au
Compare 3-2: https://llvm-compile-time-tracker.com/compare.php?from=2b7b0e178259a910355631d7d648c89052000872&to=89490929c34f45842df1588cf78d836f51c2c222&stat=instructions%3Au

Author: OCHyams

llvm/CMakeLists.txt

@@ -573,7 +573,7 @@ set(LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING "DISABLED" CACHE STRING
 set_property(CACHE LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING PROPERTY STRINGS DISABLED COVERAGE COVERAGE_AND_ORIGIN)
 
 option(LLVM_EXPERIMENTAL_KEY_INSTRUCTIONS
-  "Add additional fields to DILocations to support Key Instructions" OFF)
+  "Add additional fields to DILocations to support Key Instructions" ON)
 
 set(WINDOWS_PREFER_FORWARD_SLASH_DEFAULT OFF)
 if (MINGW)