Introduces aprecompiled_header_type flag on a tool to say whether it supports precompiled headers, and flags on configs/targets that allow one to specify which header is precompiled.
This does not implement GCC precompiled headers, but the type flag will allow future expansion (the implementation will be mostly separate).
Renames SOURCE_CC to SOURCE_CPP to avoid confusion with Toolchain::TYPE_CC (which is actually the C compiler).
BUG=297678
Review URL: https://codereview.chromium.org/1207903002
Cr-Original-Commit-Position: refs/heads/master@{#336674}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3dab5fe87ddb0efe646f6ab393ab07533d75237d
0bc79121a0