Try to avoid a realloc in utf8CString

[Catfish-Man]

No description provided.

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

@swift-ci please Apple Silicon benchmark

[Catfish-Man]

No >7%-threshold wins on AS, but x86 results look potentially nice

------- Performance (x86_64): -O -------

REGRESSION                                OLD       NEW       DELTA    RATIO    
ArrayAppendGenericStructs                 1360.0    1765.0    +29.8%   **0.77x (?)**
CharacterRecognizer.ascii                 92.353    113.412   +22.8%   **0.81x (?)**
SubstringEqualString                      156.273   177.833   +13.8%   **0.88x (?)**
StringComparison_slowerPrenormal          534.828   589.643   +10.2%   **0.91x (?)**
CStringShortAscii                         900.0     974.286   +8.3%    **0.92x (?)**

IMPROVEMENT                               OLD       NEW       DELTA    RATIO    
FlattenListFlatMap                        4167.0    2411.0    -42.1%   **1.73x (?)**
UTF8Decode_InitFromCustom_contiguous      177.455   144.385   -18.6%   **1.23x (?)**
LessSubstringSubstring                    29.512    24.057    -18.5%   **1.23x (?)**
EqualSubstringString                      29.606    24.184    -18.3%   **1.22x (?)**
LessSubstringSubstringGenericComparable   29.719    24.375    -18.0%   **1.22x (?)**
EqualSubstringSubstring                   29.839    24.481    -18.0%   **1.22x**
UTF8Decode_InitDecoding                   174.8     144.273   -17.5%   **1.21x**
UTF8Decode_InitFromData                   174.636   144.333   -17.4%   **1.21x**
UTF8Decode_InitFromBytes                  177.9     147.5     -17.1%   **1.21x (?)**
EqualStringSubstring                      29.607    24.583    -17.0%   **1.20x (?)**
EqualSubstringSubstringGenericEquatable   29.5      24.5      -16.9%   **1.20x (?)**
Data.hash.Medium                          28.103    24.258    -13.7%   **1.16x (?)**
StringComparison_longSharedPrefix         241.1     210.7     -12.6%   **1.14x (?)**
NormalizedIterator_fastPrenormal          538.605   473.0     -12.2%   **1.14x (?)**
Dictionary4OfObjects                      272.4     239.6     -12.0%   **1.14x (?)**
DropLastSequenceLazy                      330.667   296.857   -10.2%   **1.11x (?)**
DropLastSequence                          329.833   296.429   -10.1%   **1.11x (?)**
DistinctClassFieldAccesses                42.571    38.5      -9.6%    **1.11x (?)**
DropLastAnySequence                       328.833   297.857   -9.4%    **1.10x (?)**
AngryPhonebook.Strasse.Small              152.867   141.625   -7.4%    **1.08x (?)**