Don't add unnecessary compiler flags to llvm-config output

Don't add unnecessary compiler flags to llvm-config output

Summary:
llvm-config --cxxflags --cflags, should only output the minimal flags
required to link against the llvm libraries.  They currently contain
all flags used to compile llvm including flags like -g, -pedantic,
-Wall, etc, which users may not always want.

This changes the llvm-config output to only include flags that have been
explictly added to the COMPILE_FLAGS property of the llvm-config target
by the llvm build system.

llvm.org/PR8220

Output from llvm-config when running cmake with:
cmake -G Ninja .. -DCMAKE_CXX_FLAGS=-funroll-loops

Before:

--cppflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include
            -D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
--cflags:   -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include
            -fPIC -Werror=date-time -Wall -Wextra -Wno-unused-parameter -Wwrite-strings \
            -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough \
            -Wno-comment -fdiagnostics-color -g -D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS \
            -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
--cxxflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include\
            -funroll-loops -fPIC -fvisibility-inlines-hidden -Werror=date-time -std=c++11 -Wall \
            -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers \
            -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-maybe-uninitialized \
            -Wno-class-memaccess -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wno-comment \
            -fdiagnostics-color -g  -fno-exceptions -fno-rtti -D_GNU_SOURCE -D_DEBUG \
            -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS"

After:

--cppflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include \
            -D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
--cflags:   -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include \
            -D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
--cxxflags: -I$HEADERS_DIR/llvm/include -I$HEADERS_DIR/llvm/build/include \
             -std=c++11   -fno-exceptions -fno-rtti \
             -D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS

Reviewers: sylvestre.ledru, infinity0, mgorny

Reviewed By: sylvestre.ledru, mgorny

Subscribers: mgorny, dmgreen, llvm-commits

Differential Revision: https://reviews.llvm.org/D55391

llvm-svn: 349068