Singleton hack of fixing static initialisation order fiasco (#154541)

https://github.com/llvm/llvm-project/issues/154528

# Brief
Indirect linking of llvm as a shared library is causing a "free()
invalid size abortion". In my case, my project depends on google/clspv
which in turn pulls `llvm`. Note that the issue does not occur when
`clspv` and `llvm` is all statically linked.

# Structure of a project which might be causing an error
[google/clspv](https://github.com/google/clspv) has been depending on
this project (llvm-project), as a static library.
My personal project has been depending on
[google/clspv](https://github.com/google/clspv) as a shared library.
So `MyProject` was linked to shared object `clspv_core.so` which is
containing `llvm-project` as its component.

# Problem
Linking `llvm-project` indirectly to `MyProject` via `clspv_core` was
causing the `free() invalid size` abortion.
> When library is all statically linked, this problem did not occur.

[This issue](https://github.com/llvm/llvm-project/issues/154528) has a
full log of the programme running with valgrind.

# Reason in my expectation
`KnownAssumptionStrings` from
[clang/lib/Sema/SemaOpenMP.cpp](https://github.com/llvm/llvm-project/pull/154541/files#diff-032b46da5a8b94f6d8266072e296726c361066e32139024c86dcba5bf64960fc),
[llvm/include/llvm/IR/Assumptions.h](https://github.com/llvm/llvm-project/pull/154541/files#diff-ebb09639e5957c2e4d27be9dcb1b1475da67d88db829d24ed8039f351a63ccff),
[llvm/lib/IR/Assumptions.cpp](https://github.com/llvm/llvm-project/pull/154541/files#diff-1b490dd29304c875364871e35e1cc8e47bf71898affe3a4dbde6eb91c4016d06)
and `FeatureMap` from
[llvm/lib/Analysis/MLInlineAdvisor.cpp](https://github.com/llvm/llvm-project/pull/154541/files#diff-26c738eb291410ed83595a4162de617e8cbebddb46331f56d39d193868e29857),
[llvm/include/llvm/Analysis/InlineModelFeatureMaps.h](https://github.com/llvm/llvm-project/pull/154541/files#diff-3b5a3359b2a0784186fb3f90dfabf905e8640b6adfd7d2c75259a6835751a6a7)
which have been placed on global scope, causing static initialisation
order ficasso when indirectly linked by `Myproject`.

# Fix trial
Changing those global instances I've mentioned ~
`KnownAssumptionStrings` and `FeatureMap` ~ to functions which return a
static variable's left value ~ `getKnownAssumptionStrings()`,
`getFeatureMap()` ~ has solved my personal problem, so I am pulling a
request of it.