chore: more header cleanup for enabling clang-tidy

[devjgm]

This change is 

[devjgm]

Related to #355

[codecov]

Codecov Report

Merging #1236 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1236      +/-   ##
==========================================
- Coverage   94.93%   94.91%   -0.03%     
==========================================
  Files         184      183       -1     
  Lines       13824    13825       +1     
==========================================
- Hits        13124    13122       -2     
- Misses        700      703       +3

Impacted Files	Coverage Δ
...gle/cloud/spanner/internal/range_from_pagination.h	89.58% <ø> (ø)	⬆️
google/cloud/spanner/mutations.h	100% <100%> (ø)	⬆️
google/cloud/spanner/testing/matchers.h	60.86% <100%> (ø)	⬆️
google/cloud/spanner/internal/polling_loop.h	91.89% <0%> (-2.71%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	81.57% <0%> (-1.76%)	⬇️
...anner/integration_tests/client_integration_test.cc	98.47% <0%> (-0.01%)	⬇️
google/cloud/spanner/database_admin_connection.h
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.37% <0%> (ø)	⬆️
google/cloud/spanner/value.h	92.9% <0%> (+0.1%)	⬆️

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