Move the constants in the precedence table into the header.

[chandlerc]

Getting the basic constants is actually quite hot in the parser, and is
spending all of its time in memory stalls due to touching the call stack
just to return a constant when these are out-of-line.

This alone is worth another 8% improvement in parsing, and 2% in syntax
checking:

name                                               old cpu/op   new cpu/op   delta
BM_CompileAPIFileDenseDecls<Phase::Lex>/256        37.7µs ± 1%  37.5µs ± 2%  -0.58%  (p=0.019 n=19+18)
BM_CompileAPIFileDenseDecls<Phase::Lex>/1024        181µs ± 1%   179µs ± 2%  -0.95%  (p=0.001 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Lex>/4096        743µs ± 1%   736µs ± 2%  -0.85%  (p=0.001 n=20+19)
BM_CompileAPIFileDenseDecls<Phase::Lex>/16384      3.30ms ± 1%  3.25ms ± 2%  -1.48%  (p=0.000 n=17+19)
BM_CompileAPIFileDenseDecls<Phase::Lex>/65536      14.2ms ± 1%  13.9ms ± 2%  -1.95%  (p=0.000 n=17+19)
BM_CompileAPIFileDenseDecls<Phase::Lex>/262144     64.7ms ± 2%  63.8ms ± 2%  -1.30%  (p=0.000 n=18+18)
BM_CompileAPIFileDenseDecls<Phase::Parse>/256      71.0µs ± 1%  65.2µs ± 2%  -8.20%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/1024      351µs ± 1%   320µs ± 1%  -9.02%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/4096     1.43ms ± 2%  1.31ms ± 2%  -8.41%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/16384    6.08ms ± 2%  5.57ms ± 1%  -8.49%  (p=0.000 n=19+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/65536    25.2ms ± 1%  23.2ms ± 1%  -8.08%  (p=0.000 n=19+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/262144    109ms ± 1%   101ms ± 1%  -6.93%  (p=0.000 n=19+19)
BM_CompileAPIFileDenseDecls<Phase::Check>/256       752µs ± 1%   744µs ± 1%  -1.07%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/1024     1.62ms ± 1%  1.59ms ± 1%  -1.88%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/4096     4.97ms ± 2%  4.85ms ± 1%  -2.43%  (p=0.000 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/16384    19.0ms ± 2%  18.5ms ± 2%  -2.32%  (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/65536    78.8ms ± 2%  76.9ms ± 2%  -2.50%  (p=0.000 n=20+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/262144    334ms ± 1%   327ms ± 2%  -2.29%  (p=0.000 n=20+20)

[zygoloid]

Looks good. (This seems to be independent of #4307, so could presumably be landed first if you wanted.)