glcpp: use pre-expansion line number for __LINE__

Fixes the following deqp tests:
dEQP-GLES2.functional.shaders.preprocessor.predefined_macros.line_2_*

It don't see the spec requiring this, but it seems to be better, as the
clang preprocessor for example has this behavior.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
13 jobs for !1476 with glcpp in 52 minutes and 3 seconds (queued for 58 seconds)
latest detached
Status Name Job ID Coverage
  Containers Build
passed debian #475903

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  Build+Test
passed meson-arm64 #475912

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passed meson-armhf #475911

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passed meson-clang #475905

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passed meson-clover #475909

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passed meson-i386 #475913

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passed meson-main #475908

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passed meson-swr-glvnd #475904

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passed meson-vulkan #475910

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passed scons-llvm #475915

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passed scons-nollvm #475914

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passed scons-swr #475906

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passed scons-win64 #475907

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