fix: made .clang-format generic for any repo

[devjgm]

As we prepare to move to a monorepo, we want all of our existing repos
to be formatted with the same .clang-format config. This config removes
any per-library specific names (i.e., I removed "spanner"). Instead,
this names the headers and directories of the common headers and
ensures that those sort after the other quoted "google/cloud/.+"
includes, which should be the project ones.

This exact same .clang-format file should work in all of our repos.

The intent is that library includes sort first, and they sort before the common includes. For example:

// file: spanner/foo.h
#include "google/cloud/spanner/blah.h"
#include "google/cloud/bar.h"
...

Part of googleapis/google-cloud-cpp#3596

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This change is 

[codecov]

Codecov Report

Merging #1454 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1454      +/-   ##
==========================================
- Coverage   95.61%   95.54%   -0.07%     
==========================================
  Files         190      190              
  Lines       15788    15782       -6     
==========================================
- Hits        15095    15079      -16     
- Misses        693      703      +10

Impacted Files	Coverage Δ
...e/cloud/spanner/testing/cleanup_stale_instances.cc	53.84% <0%> (-23.08%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	80.55% <0%> (-1.86%)	⬇️
google/cloud/spanner/samples/samples.cc	90.01% <0%> (-0.2%)	⬇️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.96% <0%> (-0.01%)	⬇️
google/cloud/spanner/keys.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
google/cloud/spanner/transaction.h	100% <0%> (ø)	⬆️
...nner/benchmarks/single_row_throughput_benchmark.cc	90.81% <0%> (+0.02%)	⬆️
...on_tests/rpc_failure_threshold_integration_test.cc	87.77% <0%> (+0.13%)	⬆️
google/cloud/spanner/internal/spanner_stub.cc	78.49% <0%> (+1.07%)	⬆️
... and 1 more

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