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[clang][driver] Only warn once about invalid library values

Authored by tbaeder on Feb 2 2021, 9:25 AM.

Description

[clang][driver] Only warn once about invalid library values

Since ToolChain::GetCXXStdlibType() is a simple getter that might emit
the "invalid library name in argument" warning, it can conceivably be
called several times while initializing the build pipeline.

Before this patch, a simple 'clang++ -stdlib=foo ./test.cpp' would print
the warning twice, -rt=lib=foo would print 6 times.

Change this and always only print the warning once. Keep the rest of the
semantics of the functions.

Differential Revision: https://reviews.llvm.org/D95915