[CSSPGO] Adjust SampleContextFrameVector to be a shorter SmallVector

It appears that some memory saving can be archived by tweaking the existing `SampleContextFrameVector` setup. Below are memory usage for two benchmarks before and after this change.

1. Medium benchmark:
   - Before:

```
    note: After computeSizeForProfiledFunctions
    note: VM: 12.50 GB   RSS: 11.02 GB
    note: After generateProbeBasedProfile
    note: **VM: 30.56 GB   RSS: 29.08 GB**
    note: After postProcessProfiles
    note: VM: 31.91 GB   RSS: 30.43 GB
```
   - After:

```
    note: After computeSizeForProfiledFunctions
    note: VM: 12.26 GB   RSS: 10.80 GB
    note: After generateProbeBasedProfile
    note: **VM: 28.76 GB   RSS: 27.29 GB**
    note: After postProcessProfiles
    note: VM: 30.12 GB   RSS: 28.66 GB
```

2. Large benchmark:
   - Before:

```
    note: After computeSizeForProfiledFunctions
    note: VM: 106.28 GB   RSS: 91.25 GB
    note: After generateProbeBasedProfile
    note: **VM: 245.19 GB   RSS: 224.30 GB**
    note: After postProcessProfiles
    note: VM: 254.96 GB   RSS: 221.58 GB
```

   - After:

```
    note: After computeSizeForProfiledFunctions
    note: VM: 105.95 GB   RSS: 91.04 GB
    note: After generateProbeBasedProfile
    note: VM: **234.29 GB   RSS: 215.39 GB**
    note: After postProcessProfiles
    note: VM: 244.03 GB   RSS: 213.45 GB
```

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D118203