Export of internal Abseil changes

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bc74316103bbda92541896f588f71c9d45bea768 by Gennadiy Civil <misterg@google.com>:

Manually fixing the BUILD.bazel files

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d41bf9ea916a0dc8c69e6ba77f58f9d55649880e by Shaindel Schwartz <shaindel@google.com>:

Minor cleanup to miscellaneous BUILD files.

PiperOrigin-RevId: 266420157

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08a8dc2cbd48d27e1115809f9ca8d178551cd66e by Gennadiy Civil <misterg@google.com>:

Internal Change
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Internal Change
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8617d58fde1ece40e4aa79eaa5e250b42d19835f by Shaindel Schwartz <shaindel@google.com>:

Internal Change
BEGIN_PUBLIC
Internal Change
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3a0fc7d48bdc70c4f7dffc219578693dea84eb2d by Derek Mauro <dmauro@google.com>:

Implement absl::string_view::at()

PiperOrigin-RevId: 266024644

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ba53a9da8ede8fe7b8971eaab6b3a1fa34763ff6 by Andy Soffer <asoffer@google.com>:

Remove forcing of optimization levels in MSVC.

PiperOrigin-RevId: 265927588

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df86f2046b54bba7da2e345040806d43470de5c0 by Shaindel Schwartz <shaindel@google.com>:

Internal change

PiperOrigin-RevId: 265811077

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e2e3a6e8194363e7c6377672560c806d638a7c74 by Derek Mauro <dmauro@google.com>:

Remove ABI unsafe mixed exceptions mode compilation.
Testing will now be done on CI with the exceptions flag set globally.

PiperOrigin-RevId: 265796079
GitOrigin-RevId: bc74316103bbda92541896f588f71c9d45bea768
Change-Id: Ibccd00f4829520454aa55c4f55c7cb2dc9c6b65a