Migrate all CARBON_VLOG to the format string variant.

[chandlerc]

This mostly uses a hilarious set of regular expressions to mechanically
switch all but two uses, and then manually fixed the last two. There
weren't too many.

Also simplifies the vlog implementation now that it's all going
through a format string.

This alone has a nice impact on parse and check of about 2% and 1%
respectively. The impact on lex in my timings looks like noise (no
change in instruction count, unlike the other phases).

name                                               old cpu/op   new cpu/op   delta
BM_CompileAPIFileDenseDecls<Phase::Lex>/256        39.1µs ± 3%  38.1µs ± 2%  -2.42%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Lex>/1024        187µs ± 3%   183µs ± 1%  -2.30%  (p=0.000 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Lex>/4096        776µs ± 4%   756µs ± 1%  -2.62%  (p=0.000 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Lex>/16384      3.36ms ± 1%  3.33ms ± 1%  -0.90%  (p=0.000 n=18+18)
BM_CompileAPIFileDenseDecls<Phase::Lex>/65536      14.4ms ± 2%  14.2ms ± 1%  -1.41%  (p=0.000 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Lex>/262144     65.7ms ± 1%  65.2ms ± 2%  -0.86%  (p=0.002 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/256      87.5µs ± 1%  86.3µs ± 1%  -1.43%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/1024      438µs ± 2%   431µs ± 1%  -1.54%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/4096     1.81ms ± 2%  1.77ms ± 1%  -2.12%  (p=0.000 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/16384    7.54ms ± 1%  7.43ms ± 1%  -1.44%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/65536    31.2ms ± 1%  30.6ms ± 1%  -2.03%  (p=0.000 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/262144    133ms ± 1%   130ms ± 1%  -1.85%  (p=0.000 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/256       882µs ± 1%   878µs ± 1%  -0.52%  (p=0.001 n=17+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/1024     1.90ms ± 2%  1.88ms ± 1%  -1.17%  (p=0.000 n=19+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/4096     5.85ms ± 2%  5.76ms ± 1%  -1.43%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/16384    22.2ms ± 2%  21.9ms ± 2%  -1.20%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/65536    91.2ms ± 2%  90.3ms ± 1%  -1.00%  (p=0.000 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/262144    382ms ± 1%   380ms ± 1%  -0.51%  (p=0.003 n=18+19)